Scott Morrison said the booster program was essential if Australia was to avoid a new wave of virus infections.
"We're writing out to all Australians and also encouraging them to now go and get their booster shots," Mr Morrison told ABC Radio Adelaide on Friday.
"What you're seeing over in Europe at the moment with a further wave, (booster shots) are very important. We keep rolling on."
Booster shots are available for anyone who received their second COVID-19 vaccine six months ago or more.
While Pfizer is the only vaccine manufacturer that has had their booster dose approved, it can be administered to anyone regardless of what brand they received for their first or second dose.
The letters urging people to take up the booster program will be sent to 11.3 million homes, and are expected to arrive before Christmas.
The head of Australia's vaccine rollout, Lieutenant-General John Frewen, said 370,000 people have already received their third dose of a COVID-19 vaccine.
However, more than 500,000 people have become eligible for the top-up shot.
"The primary two doses are the most important thing, and we'll keep working on getting everybody across the line on those," Lieutenant-General Frewen told ABC TV.
"But for everybody else, we strongly encourage you, if you're six months from your second dose, to get out and grab a booster."
The booster push comes amid a new COVID-19 variant being detected in South Africa, which has alarmed health officials.
The new variant, dubbed the Nu strain, has double the number of mutations as the Delta strain which led to lockdowns in multiple jurisdictions in Australia
The latest Australian vaccination figures have shown 86 per cent of the national population over 16 are fully vaccinated, while 91.9 per cent have got their first dose.
Most of the people who are now eligible for their booster shot were among the first to get the COVID-19 vaccine when the rollout started, including healthcare workers as well as aged care and disability care residents and staff.
While supply problems plagued the start of the vaccine rollout, Lieutenant-General Frewen said it was not an issue for the third doses.
"There is absolutely no constrain on supply now and people will be able to use all sorts of pathways," he said.
On Friday, there were 1362 new COVID-19 cases in Victoria with seven deaths reported.
Meanwhile, in NSW there were 261 infections with no fatalities.
There was just one new case in the Northern Territory reported on Thursday, after the Top End experienced a recent spike in infections, while the ACT had eight cases.