Buying a basket of Aldi groceries is $18 cheaper when compared with the same items sold at Coles and Woolworths, according to new Choice research, which again found little price difference between the two major grocers.

As part of its quarterly supermarket survey, Choice sent mystery shoppers to compare prices of 14 common grocery items in 104 supermarkets across Australia in June.

The group was commissioned by the federal government to conduct the surveys every three months for three years in an effort to provide better transparency of the sector.

It delivered its first report in June, finding Australians were paying 25 per cent less on items at Aldi than the big grocers.

In its second report released on Thursday, Choice said the average basket of groceries without specials purchased at Aldi cost $50.79, which is much less than the same basket of goods sold at Australia's two biggest supermarkets.

The same type of items at Woolworths set customers back $68.70, while Coles cost an average of $69.91.

Choice's report found there was little price difference between the two major chains, though it had widened from $0.75 in March to $1.21 in June.

The report comes fresh on the heels of the Australian Competition and Consumer Commission's court action against the supermarket giants over misleading claims about discounts on hundreds of products.

The sector has been under a cloud for more than a year as households have struggled with the cost of living, which has prompted accusations of price gouging and a series of high-profile inquiries.

"I think [the report] just confirms what we already knew, which is that Aldi tends to be cheaper," said Mark Serrels, the editorial director of Choice.

"And [it] also confirms something else that I always find really surprising, is there are small differences, but for the most part, Coles and Woolies are almost identical [on prices], which is, in and of itself, quite telling.

A Woolworths spokesperson said the grocer "remains focused on delivering value for customers and continues to work harder to help them easily find that value when shopping our stores or online".

"We also know our own brand products are very competitive with the prices at discount retailers," they told the ABC in a statement.

IGA and Coles did not respond to requests for comment before publication.

But the Australian Retail Association chief executive Paul Zahra has called for more transparency around Choice's selection of items and said "there are a number of data discrepancies that require clarification".

He said comparisons between premium national brands at Coles and Woolworths and Aldi's private label are "not a fair or useful comparison for customers".

Choice said comparable items were chosen based on a range of factors including ingredients, country of origin and packaging similarities but said exact brands and items are not publicly revealed to prevent the study being "compromised".

The differences in prices put on 'special'

Choice found the cost of its basket of items without specials increased at Woolworths by $0.12 and at Coles by $0.59 between March and June. At Aldi the price dropped by $0.72.

The group also tracked the price differences when specials were applied to 14 items at Aldi and the major supermarkets as well as IGA.

Aldi was still the cheapest at $50.79, with Coles $16 higher at $66.22, though it overtook its other major rival to claim the second spot.

Woolworths' basket of goods on special totalled $68.37, while the items at IGA were up around $78.95.

Choice noted that when items including special prices were compared, the overall cost of the basket at Woolworths increased by $3.44 between March and June, and at Coles it dropped by $2.30.

"It feels that there's this massive movement right now for people super upset at Coles and Woolworths, so I thought there may have been pressure there in response with lower prices, but no," Mr Serrels said.

Choice said that due to problems determining the full price of special items at IGA stores, they were unable to calculate a figure for their full-priced basket.

"Because IGA is a franchise, it doesn't have quite the same purchasing power as the massive supermarkets, like Coles and Woolies," Mr Serrels said of IGA's higher prices.

"They're almost like corner shop-style supermarkets. So things tend to be more pricey there."

Jordan Lack, managing director of ALDI Australia, said the consumer group's survey "reinforces our position as Australia’s most affordable supermarket".

"We've seen more and more customers turning to ALDI for our everyday low prices," he told the ABC in a statement.

It pays to shop around

As part of the feedback to its last survey, Choice introduced a spotlight item, comparing home brand prices on laundry powder or a similar national product in the case of IGA, which doesn't have a homebrand detergent.

"Singling out individual items and having discussions around that, I think would be quite useful," Mr Serrels said.

The consumer group found Aldi had the cheapest product, but only by $0.01. Its washing powder was $5.49 compared to the Woolworths product that cost $5.50.

The Coles product was $6.20, and the national brand product at IGA was $10.30 on average, based on the prices recorded at stores across the country.

Mr Serrels said one of the most important takeaways from the report was that consumers needed to be savvy when it came to shopping around for discounts.

Earlier this week, the ACCC accused Coles and Woolworths of breaching consumer law with promotions about discounts that were "illusory".

Rather than items being lower, it found some advertised prices were actually higher.

"We always recommend to take a good close look at the unit pricing, which is on every tag [and] will tell you you're [whether you're] paying 20 cents per tea bag or $2 per kilo," Mr Serrels said.

"That's the most effective way to check if something's actually a good deal or not."

The Choice report again found Australians are paying different prices depending on where they live, though there wasn't much price difference between capital cities and regional areas that had access to all three stores.

What it did highlight was that some states are benefiting from more supermarket competition than others.

"I think competition creates a price pressure, and that's a big problem here in Australia," he said.

"When you go to a country like the UK, it's not just Coles and Woolies. There's four or five Coles and Woolworths-sized supermarket chains there, and that drives the prices down a bit."

Choice is set to deliver its next survey in December.