The Royal College of Pathologists of Australasia says third doses are particularly low in Queensland and NSW even as COVID-19 cases rise.
"With winter commencing, it is important for everyone that they are fully up to date with all relevant vaccinations," RCPA fellow Professor William Rawlinson said.
"The RCPA recently highlighted that it is very likely that we will experience far more influenza cases in Australia this winter. This, combined with the current, rising trend of COVID-19 cases, is likely to put an extraordinary strain on the healthcare system."
Western Australia has the highest uptake of third doses about 80 per cent, while Queensland is the lowest at 58 per cent. Nationally, about two-thirds of eligible Australians have received a booster.
On Wednesday, the Australian Technical Advisory Group on Immunisation expanded eligibility for a second booster to people with health conditions or a disability.
Previously, the fourth dose has only been available to those 65 and over, those in aged or disability care, people who are severely immunocompromised or Indigenous people aged over 50.
Acting Health Minister Katy Gallagher urged eligible Australians to get their fourth shot.
LATEST 24-HOUR COVID-19 DATA:
NSW: 10,926 cases, 30 deaths, 1204 in hospital with 38 in ICU
Victoria: 12,421 cases, 19 deaths, 559 in hospital, 34 in ICU
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