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.