Australia's Covid-19 booster vaccination program begins today, meaning anyone over 18 who received their second dose at least six months ago - 8 May - can book to have a third shot.
Today marks the formal beginning of the booster program, but more than 173,000 boosters had already been administered in Australia as of Saturday.
The Pfizer vaccine is currently being offered for booster shots, even for people who were vaccinated with the AstraZeneca jab.
While the government's expert advisory group on vaccines said Pfizer was the preferred shot, AstraZeneca could be used as a booster for people who received it as their primary course, or if they had an adverse reaction to an mRNA vaccine.
Moderna vaccines have been bought to be included in the booster program, but the company is awaiting finally approval by the medical regulator.
The government has said people will not be required to receive booster shots in order to be exempt from Covid-19 restrictions.
Australia's Health Minister Greg Hunt said while two doses of a Covid-19 vaccine provided "very good protection, especially against severe disease", the vaccine's ability to prevent transmission waned over time.
"A booster dose, six or more months after the second dose, will make sure that the protection from the first doses is even stronger and longer lasting and should help prevent spread of the virus," Hunt said in a statement.
He said there was no global view yet as to whether a fourth or annual vaccines would be needed.
"The epidemiology and history still has to be determined on that," he said.
He also provided an update on the government's plans to vaccinate children aged five to 11, saying it was unlikely to happen this year.
The medical regulator, the Therapeutic Goods Administration, has not yet given the green light to any Covid-19 vaccine for young children.
Having gained approval in the United States, Pfizer has applied for its vaccine to be approved for use in that younger age group in Australia but Hunt said the regulator was still waiting on the company to submit all its paperwork.
"We haven't got the full detail yet from Pfizer," Hunt told Sky News.
"And Moderna will also put in its application. So our plan and expectation has always been to commence as soon as they have completed their deliberations. But those deliberations are going to take the coming weeks."