More than half a million Australians are dealing with COVID-19 infections as authorities begin to roll out a second booster for vulnerable groups.

Experts say a fourth jab will be critical in the effort to protect at-risk Australians ahead of winter, with a surge in cases of the virus and influenza looming.

The number of active cases in Australia has climbed above the 500,000 mark for the first time since late-January, when the initial Omicron wave receded.

Around one-in-five PCR tests are coming back positive in Western Australia which is still in the midst of its first major outbreak.

People aged 65 and older, Indigenous Australians aged at least 50, disability care residents and the immunocompromised are among those able to receive their fourth dose from Monday.

An estimated 4.7 million people will be eligible to get a fourth dose but it is expected fewer than 200,000 will meet requirements at the start of the rollout.

People can have a second booster shot four months after receiving their first.

"The issue is the pandemic is not over," infectious diseases physician Peter Collignon told AAP.

"At the moment it does appear for the very vulnerable groups there is a benefit (of the booster).

"The decline in antibodies over time is what the booster doses address, but at the moment there is limited data on whether it reduces the number of infections, hospitalisations and deaths.

"We do know the longer the doses are spaced, the better the antibody protection and there's no doubt if you're an adult you need to be vaccinated with at least two doses."

Professor Collignon expects further data to be available in the next three months, as countries like Israel and the United Kingdom complete their fourth doses.

But it is important for the world to focus on making sure all countries are able to vaccinate their populations to reduce the risk of the virus mutating further, he said.

"Global vaccine access is imperative ... if we don't make sure other countries are vaccinated it puts everyone's progress and protection at risk," Prof Collignon said.

The Australian Health Protection Principal Committee expects an infection peak will hit in mid-April in several jurisdictions.

Comprised of chief health officers from across the country, the group says it is considering recommending the removal of quarantine for COVID-19 close contacts.

It says isolation could be replaced by frequent rapid antigen testing, mask wearing outside the house and limiting access of close contacts to high-risk settings.

Australia reported more than 45,000 new COVID-19 infections and 14 additional deaths with the virus on Monday.

LATEST 24-HOUR COVID-19 DATA FROM ACROSS AUSTRALIA:

NSW: 15,572 cases, six deaths, 1418 in hospital, 56 in ICU

Victoria: 10,011 cases, one death, 305 in hospital, 17 in ICU

Tasmania: 1725 cases, no deaths, 40 in hospital, one in ICU

Queensland: 7707 cases, one death, 437 in hospital, 14 in ICU

ACT: 739 cases, no deaths, 45 in hospital, 4 in ICU

Western Australia: 6082 cases, six historical deaths, 231 in hospital, six in ICU

Northern Territory: 304 cases, no deaths, 14 in hospital, one in ICU

South Australia: 4595 cases, no deaths, 209 in hospital, eight in ICU