After months of relative calm, many Australian states are once again experiencing a new wave of COVID-19.

The fourth wave was first signalled in late October when Victorian Chief Health Officer Brett Sutton indicated a 25 per cent increase in cases week-on-week.

This was echoed days later by his New South Wales counterpart Kerry Chant, who said people should prepare for a rise in cases.

Last week, Queenslanders were urged to mask up in certain situations after the state's traffic light COVID-19 advisory system switched from green to amber, due to an increase in active cases and hospitalisations

In the first week of November, South Australia saw the highest number of cases reported in the state since early September, when it was bouncing back from a winter wave.

And, despite Northern Territory Chief Minister Natasha Fyles announcing that the territory's remaining chief health officer directions would be lifted, the Aboriginal Medical Services Alliance NT says it is "concerned" about the change, due to rising cases there.

On Tuesday, Health Minister Mark Butler said the federal government would roll out Pfizer's new bivalent vaccine as a booster shot for Australians aged 18 and over.

Moderna's bivalent shot was approved for use in September.

Here's what you need to know about Australia's developing fourth COVID-19 wave.

When is the fourth COVID-19 wave expected to peak?

Queensland Chief Health Officer John Gerrard has said the previous three waves peaked between five and six weeks from their onset.

"That would put a peak somewhere around the 12th or 19th of December," Dr Gerrard said.

However, information from overseas suggests this wave could be shorter and sharper, with a peak closer to the beginning of December.

Should I be worried about new COVID-19 variants in Australia?

The Omicron variant is still the most prevalent type of COVID-19 in Australia, with several sub-variants bubbling up over the past few months.

While the big one so far — BA.5 — might be on the decline, it still makes up most cases being reported in Australia.

BA.2.75 — which was first detected in India in May and was nicknamed Centaurus — is still kicking around too.

However, two new sub-variants – BQ.1 and XBB – have been increasingly observed in Australia.

Overseas data suggest XBB and BQ.1 transmit faster than the currently reigning BA.5, but signs show that severity does not increase and symptoms remain largely the same.

During Singapore's recent XBB wave, for instance, while case numbers rose, hospital admissions did not follow suit.

What are the isolation rules around the country?