If you were to grab a box of Cadbury Favourites off the supermarket shelf, how much would it cost?

It depends on whether you're at Coles or Woolies, and what week it is.

At Coles, a single box costs either $11 or $22.

And at Woolworths, it's the same, but on different weeks.

It ends up looking something like this, where the price repeatedly moves up and down between two points.

If you don't feel like tucking into a box of choccies, how about an Australian staple — Arnott's Tim Tams. 

They follow a similar pattern, with the price dropping every other week — except for Coles, where the price stays high for two weeks in July.

You might have an idea about how supermarket specials work — there's a retail price, and if there's extra stock or a special promotion, there's a discounted sale price.

But for a thousands of products at Coles and Woolworths, like the box of Cadbury Favourites and the packet of Tim Tams, these specials aren't occasional. They follow a clear, and sometimes predictable, up-and-down movement.

The ABC analysed the online prices of nearly 44,000 items at Coles and Woolworths available to purchase between the end of May and mid-August this year, not tied to any specific location.

The analysis assumes the shopper is taking one product off the shelf, and so it excludes "multi-buy" specials, such as two-for-one.

It found that roughly 2,500 products moved in weekly cycles, mostly alternating between two or three price points, just like the box of Cadbury Favourites.

A majority of these are pantry items, like chocolates and soft drink, but products from all parts of the supermarket are included.

We've picked out a few products to show you what this price movement looks like.

These items are part of a much larger group of products — around 22,000 — that moved in only an "up, down" motion, at least four times, within a 12-week period.

That's about about half of all products analysed.

Compared with the earlier example, most of these products don't drop in price every week, but maybe every second or third week. 

Here are some examples.

These products are roughly split evenly between the two major supermarkets. About 48 per cent of these products are from Woolworths, and 52 per cent are from Coles.

While the timing of the price drop may be different, what unites them all is that they move from highs to lows, with the low price often accompanied by a bright yellow "specials" ticket.

It's led the Australia Institute's chief economist Greg Jericho to ask what the real price of supermarket items are.

"There's no way of understanding and no way of knowing," he said.

"It's really showing that prices are not being set due to demand and supply."

Internet plug-ins reveal price patterns

The ABC got the supermarket data from Adam Williamson.

He works as a data professional and began collecting the daily prices for each product at Coles and Woolworths in his spare time earlier this year.

Online supermarket data gives a pretty close indicator to what's happening in stores, with the biggest differences likely in remote areas and for perishable items.

You can see the data for yourself through Mr Williamson's two internet browser extensions, called Coles Trend and Woolworths Trend.

Once downloaded, you can see the 10 most recent price changes for any product on the Coles or Woolworths website in a bar chart.

The idea of price cycles first landed with Mr WIlliamson when he was looking at a specific product in the data.

"It's a funny one, because it's not a product that I use that got me interested — it was platinum blonde hair dye," he said.

"It was one of those products that was at half price, and it just blew me away that one week it's $16 and the next week it's $8 and next week it's $16 … and I thought, how often is that happening?"

He realised it wasn't a one-off, and started sharing his discoveries on TikTok as "Price Check Guy".

"That first post has quickly got a quarter of a million views, and it made me realise people were interested in this subject matter as much as I am," he said.

"So I've kept posting and sharing insights around prices.

"It's a team effort, sort of collectively trying to better understand pricing in supermarkets."

The thinking behind high-low price changes

Sanjoy Paul, an associate professor in operations and supply chain management at the University of Technology Sydney's business school, said these price movements were a retail strategy called "high-low" pricing.

Retailers set an initial price high, and then they discount it, and then go back to the original price.

They do so repeatedly as a way to drive appeal for a product.

"In the case of high-low pricing, demand would be high during the discounted period, and demand would be lower when the price goes back to the initial price," he said.

Across both Coles and Woolworths, the median price drop for all items was 28 per cent of the product's standard price.

And to help out with grocery shopping, we also found that these high-low products were most likely to drop in price on Wednesdays, and to a lesser extent, on Sundays.

But when we took a look at the various categories of products, we began noticing some differences. 

We divided the high-low supermarket products into 13 categories, guided by the supermarket's own categorisation of the items.

The biggest chunk of high-low products were pantry items, such as chips, lollies, chocolate, sauces and canned food.

These products were followed by beauty and personal care items, which include vitamin supplements, medicine and makeup.

Perishable items — including fruit, vegetables and bread — held the smallest proportion, which can be see below.

The amount of discount is different depending on the product, too.

We can see it in the below chart, where the taller the peak, the higher proportion of products. 

So for beauty and personal care items, most products dropped about 40 to 50 per cent, as this is the section with the highest peak.

This chart can then be extended across all categories, so you can see the proportion of discounts across all the items.

Most non-alcoholic drinks and pantry items tended to fluctuate between 20 and 40 per cent.

The lowest percentage drops were for fresh produce, where most drop by less than 20 per cent.

Kathryn Backholer, a professor of public health policy at Deakin University, is part of a team that has analysed online supermarket data for a few years, and has observed a similar trend.

Her team found there were price discounts on less nutritious foods, like snack foods, more frequently and to a bigger extent than for healthier options such as fruit and vegetables.

Dr Backholer said through interviews with shoppers, they found this pricing practice disincentivised people from buying healthy food.

"From both the quantitative and qualitative data, people are more likely to purchase via an impulse buy," she said.

Sales prices condition consumers

Dr Jericho said this pricing strategy had "groomed" consumers, locking them into shopping at Coles and Woolworths.

"What they're really doing is ensuring that customers always know they can go to Woolies or Coles and to get Coca-Cola on special — they just have to know which week it is," he said.

While the ABC has not investigated each product's cycle individually, the price drops appeared to roughly alternate between the two major supermarkets for dozens of items.

Dr Jericho said the fact that some discounts appeared to be in lock-step between Coles and Woolworths meant small and independent retailers were consistently competing with the sale price.

"Every week there's a sale price on that item, and so if you're not in the game, you're not in the pattern," he said.

"You're forced to have to always be competing, in a sense, with the sale price, rather than the normal price, because Coles and Woolies are able to use their power to take it in turns."

Dr Jericho said it looked like the two supermarkets were colluding, which is illegal under Australian consumer law, but admitted this was a "high bar to prove".

The Australian Competition and Consumer Commission (ACCC) points out on its website that similar pricing between competing businesses does not indicate collusion.

"This can create price changes that may look like price fixing … however, this is unlikely to be illegal as long as each business is making independent decisions about its prices."

A Woolworths spokesperson said the timing of promotions for weekly specials was typically initiated by suppliers, and it did not have visibility of Coles' future pricing.

“The timing of promotions is influenced by our suppliers who need to have adequate stock to meet higher demand over the promotional period," they said.

“We do not have visibility of our competitors’ future pricing or promotional plans."

A Coles spokesperson said there were many factors which came into whether a product went on special, and the intention was to always give the customer the best deal.

"With more customers telling us they seek out specials and discounts when they shop, we work very closely with our supplier partners to make investments and help lower the prices of items shoppers buy most," they said.

"There are many factors which help us decide if and when to put items on special, including seasonality, availability, supplier preference, but also the cadence with which our customers most commonly purchase products so they can access great value prices when they need it."

Dr Paul said suppliers did have a role in setting prices, but said supermarkets were likely to hold the power in pricing negotiations. 

"In Australia, the two major supermarkets have 67 per cent market share, and so it means suppliers don't have many choices here," he said.

However, Dr Paul said there was no way to know for sure, as there was no transparency around how the two major supermarkets priced their products.

"Consumers don't understand whether it's a real discount or not, and it's impossible for them to track the historical pricing data," he said.

The ACCC investigates the two major supermarkets

The government has made significant moves to investigate the country's supermarket duopoly after they recorded surging profits amid a cost-of-living crisis.

It ordered the ACCC to investigate various elements of supermarket pricing across the sector as part of a 12-month inquiry. The final report is due in February 2025.

While the ACCC said it was not able to comment specifically on high-low pricing as the inquiry was underway, it pointed the ABC to the ministerial direction for its investigation into supermarkets.

The direction said the ACCC would look into the "competitiveness of small and independent retailers" and the "approach of suppliers, wholesalers and retailers to setting prices for groceries". 

The commission's interim report was released last month, which found consumers don't trust "sale price" claims from the two major supermarkets.

Separately, the ACCC has taken legal action against Coles and Woolworths, accusing them of allegedly misleading consumers with untrue discount pricing.

They allege the two supermarkets had hundreds of products at a stable price, increased them, and then decreased them to a point higher than the initial price and advertised them as "Prices Dropped" and "Down Down" specials.

The products affected by this legal action are separate to the weekly specials products analysed by the ABC.

Dr Jericho said the government needed to give the ACCC permanent powers, or create an entirely new body, to monitor supermarket products over time to investigate why their price fluctuated.

While the ACCC is currently looking at prices for the supermarket inquiry, the commission ordinarily does not keep track of them.

"The ACCC is really hamstrung — they're not looking at prices, which is really the main game," Dr Jericho said.

Dr Backholer agreed and said more transparent price-setting practices, as well as breaking up the concentration of power in the supermarket sector, would support public health efforts and lower prices.

In the meantime, Mr Williamson — the creator of Coles and Woolworths Trend — said the main goal of his work was to promote more transparency.

He referenced consumer laws in the EU, which makes it compulsory for retailers to tell consumers about the lowest price applied to the product in the previous 30 days.

"Ideally, I'd love shoppers to have that price history available to them on the ticket that shows the last 10 sales cycles, so shoppers know that they are getting a real, genuine discount."

Mr Williamson said he had attempted to get in touch with Coles and Woolworths about incorporating his extensions into their website.

He holds hope they would consider it, so he didn't have to keep running the software.

"That's my end goal — success for me is shutting down the plug-ins."

Data notes

• The data obtained for 26 May 2024 to 17 August 2024 inclusive covers a 12-week period containing an entry for every product captured by Coles and Woolworths Trends for the respective supermarket website each day. The data was scraped from each website's site-map, so products with an active URL were captured.
• The product's price and availability was collected. Any product that was available for less than 70 days was omitted from the analysis.
• Four extreme outliers — where the price moved more than 20 times in the given period — were omitted from the analysis.
• Online prices are understood to be largely the same as in-store prices, with differences depending on location and perishable items. Online-only specials have been included in the analysis, which represent a very small proportion of the total products analysed.
• Both the Coles and Woolworths data had between 2-5 days missing for various products. It was assumed the products affected were unavailable on that day.
• Cigarettes were removed from the category analysis as there were only three products represented.
• The recorded price change may not have happened on that exact day listed, but on a day or two earlier due to the timing of the data collection.
• Not every line of the dataset was validated, but it was cross-checked against open-source supermarket price platform HotPrices.org for its available data. The method was validated after the fact with a manual spot check of more than 200 products across four weeks after the analysed period. The platforms Coles Trends and Woolworths Trends are also public facing and have been open to scrutiny since July.