The nation's expert vaccine advisory body has recommended that anyone older than 50 receive a fourth dose of the COVID-19 vaccine.
Health Minister Mark Butler said 7.4 million more people will now be eligible for the fourth dose from July 11.
The Australian Technical Advisory Group on Immunisation (ATAGI) specifically recommended the shot for people over 50, while people aged 30 to 49 may choose to have a fourth dose.
ATAGI reiterated those already eligible, including those over 65, remain at high risk of severe disease and death from COVID-19.
Federal Health Minister Mark Butler said there was "a strong case" for Australians under 65 to receive the additional booster shot but said there was an "open question" about whether young, healthy people needed it.
ATAGI did not support making the fourth dose available to healthy adults under the age of 30 years as it was not clear whether the benefits outweighed the risks in this population group.
Earlier this week, NSW Health Minister Brad Hazzard said he would back a general expansion of the vaccine rollout while Victorian Premier Daniel Andrews urged the fourth shots to be given to healthcare workers.
The fourth dose was already available to people aged over 65, aged care residents, Aboriginal and Torres Strait Islander people aged 50 and older, people with a disability, the immunocompromised and anyone over 16 with a medical condition that increases the risk of severe COVID-19.
On Wednesday, Prime Minister Anthony Albanese said he had urged health experts to work quickly on matters relating to COVID-19, including vaccines.
"I've encouraged them, when I met with the health authorities, including the CMO, before I departed a bit over a week ago, I encouraged them to do things as quickly as possible," he said on Wednesday.
Boosters slump as new COVID wave picks up
ATAGI noted the number of people ill from the virus had increased in recent months, placing an increased strain on hospitals.
It said new Omicron subvariants had exacerbated the number of illnesses, and this was expected to worsen in coming months.
Last week, Australia had more COVID cases per capita than any G7 nation except France, according to the University of Oxford's Our World In Data project.
While more than 95 per cent of people over 16 are fully vaccinated, the number of people getting boosters has slumped since February.
Only 70.5 per cent of the eligible population has received three or more doses.
About 2,413,000 people had received a fourth dose as of Monday, 59 per cent of people who were eligible.
"My message to everyone living in Australia aged 50 and over is to make sure you have the greatest protection against COVID-19 by having a fourth dose as soon as possible," Mr Butler said.
"If you are aged 30 to 49 and you want that extra protection, you can choose to get a fourth dose."
Mr Butler said two doses were "simply not enough", and even three was insufficient for older people.
People who have recently been infected with the SARS-CoV-2 virus are advised to delay their booster shot until three months after their infection.
Australian Medical Association vice-president Chris Moy said ATAGI had been waiting for the appropriate time to expand eligibility.
"They have been pressured for a long time now to go to a fourth shot for everybody, but they wanted to hold their shot," Dr Moy said.
Dr Moy — who is a technical expert with ATAGI but not a voting member involved in the decision — questioned opening a fourth booster to those older than 30.
"The evidence for that group is not super high," Dr Moy said.
"It's more important to get that first booster.
"There isn't strong evidence for that except for a transient reduction in getting infection and transmission [down]."
