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.
“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.
It 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 the Omicron variant
It said it made the changes as the pandemic had “significantly changed in recent weeks”.
“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.
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.”
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.