Initially four months between jabs, that has been reduced to three months now as the Omicron variant continues to grip the nation.

That means, if you had your second dose of COVID-19 vaccine three months ago, you'll be eligible for a booster shot by the end of the week.

Already more than 50 per cent of eligible people have so far had their booster in NSW alone but now millions more will become eligible from Friday.

"In our 40 vaccination hubs across NSW, we will be bringing forward the eligibility for a booster shot from four months to three months," NSW Premier Dominic Perrottet said.

Bringing the interval forward would enable more than 3.5 million more people in the state to become eligible for the booster.

The state hubs have the capacity to deliver about 250,000 vaccinations every week.

In the ACT the decision means an extra 183,000 Canberrans will be eligible for the COVID-19 booster.

Victorian Premier Daniel Andrews announced the change on Wednesday and said it came into immediately effect.

"The total number of people eligible increases substantially by two million Victorians and it will help us get more people third-dose boosted quicker than would otherwise be the case," he told reporters in Melbourne.

Some 60,000 additional appointments will be available as part of a four-day "booster blitz" beginning on Friday at eight of the state-run vaccination hubs.

The Pfizer and Moderna mRNA vaccines are recommended as boosters in Australia. Although not preferred, the AstraZeneca vaccine can be given as a booster for those who are unable to have an mRNA vaccine for medical reasons.

COVID-19 booster vaccines are not mandatory. They are highly recommended to continue to protect you, your loved ones and your community against COVID-19.