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

The number of active cases 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.

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.

People aged 65 and older, Indigenous Australians aged at least 50, disability care residents and the immunocompromised are among those receiving 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 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.

More than 47,000 new COVID-19 infections and 17 virus-related deaths were reported across the country on Sunday.

Almost 2600 patients are being cared for in hospital wards, 103 of them in intensive care.

– AAP