Australians will be able to get their COVID-19 vaccine booster shots after four months from early next year, and then three months from the end of January.
Health Minister Greg Hunt said the changes, based on ATAGI advice, will come into effect from January 4.
"The planning behind that is it will open up a new cohort," Mr Hunt said.
READ MORE: Masks return inside for Christmas in NSW
"Approximately 7.5 million people will become eligible on the fourth of January."
From January 31, the timeframe will be shorted even further to three months.
The interval had already been shortened from six months to five.
ATAGI, Australia's medical body, said there was "strong evidence" boosters would help increase protection against infection with the Omicron variant.
https://twitter.com/jekearsley/status/1474147886803795968It said superspreading events in NSW such as the outbreak in bars and clubs in Newcastle indicated two doses "did not provide any significant protection" against Omicron.
It said it made the changes as the pandemic had "significantly changed in recent weeks".
READ MORE: 'Perfect bottleneck' hampering Australia's vaccine booster rollout
"Strong evidence has accumulated over the past two weeks to indicate that booster doses of COVID-19 vaccines are likely to increase protection against infection with the Omicron variant," ATAGI said in a statement.
"Although some early data suggest that the risk of hospitalisation due to disease caused by the Omicron variant is lower than that with the Delta variant, this difference would not be enough to offset the impact of high case numbers on the health system."
Third vaccine doses will be prioritised with preference placed on people who have the bigger gap in time between doses, and the immunocompromised.
READ MORE: People wait all night for COVID-19 tests
Australia's Chief Medical Officer, Professor Paul Kelly, also said concerns around Omicron's transmissibility fuelled the changes.
"What we're seeing in other parts of the world is a doubling rate of cases every two or three days, and that is what we are seeing here," Professor Kelly said.
"We're not seeing that increase in hospitalisation or ICU.
"Make that booking in the New Year to get that booster if you're becoming eligible. If you're eligible now, don't hesitate."
"While this is a more transmissible disease it does appear to be less severe," Mr Hunt added.
"We're taking these precautions to add extra protection for Australians and to help them stay ahead of the curve."
READ MORE: What you need to know about the booster rollout
ATAGI has given individual jurisdictions power to move forward with their planning if they have the capacity, as long as they don't impact more eligible citizens.
"As an example, I spoke to the Premier of South Australia last night," Mr Hunt said.
"He believes they have very strong capacity, so ATAGI has given the jurisdiction freedom to move forward."
Mr Hunt explained dates have been moved forward "out of an abundance of caution" as Omicron spreads.
"The protection as it is very strong against severe illness, but what we will see is a much stronger protection against transmission," he said.
Mr Hunt said there's ample vaccine supply because the government has prepared for possible fourth and fifth booster shots, though medics had already warned of a distribution "bottleneck".
"We have 20 million vaccines that are in the country now," he said.
"More than five million are in the field on the basis of orders."
In addition, 60 million Pfizer doses and 15 million Moderna have been ordered for 2022, as well as an extra 51 million doses of Novavax, which still needs TGA approval.
"We're anticipating there may need to be third and a fourth dose and indeed there is enough with those figures for five doses," Mr Hunt said.
"We're in a fortunate position... whether a fourth or fifth dose is required we just provisioned on the absolute highest-need, worst-case scenario."
