Australia’s COVID booster rollout will be ready, if medical experts advise any changes due to the emerging Omicron variant, Prime Minister Scott Morrison says.

While people who are fully vaccinated must currently wait for six months until they are eligible for their booster, medical experts are examining whether the timeframe between doses could be shortened.

Scott Morrison said authorities were prepared should advice surrounding booster shots change.

“[Health officials] will continue to keep that under close watch as to whether [boosters] can be brought forward with new information,” Mr Morrison said in Geelong on Thursday.

“We are ready to go if that is what they would like to do.”

Across the country, more than 600,000 people have received COVID-19 booster shots since they were first approved.

As more cases of the virus linked to the Omicron variant are detected, Mr Morrison said a third dose of the vaccine could be critical.

“Australia is one of the first countries to introduce a whole-of-population booster program, and we did that before Omicron,” he said.

“The events only show the wisdom of that decision. We have ample booster shots.”

Australian medical regulators have also granted provisional approval for the Moderna vaccine to be used as a booster.

A final decision on approval from Australia’s leading vaccine body is expected next week.

The news came as Queensland and the Northern Territory became the latest jurisdictions to pass 80 per cent vaccination rates.

Western Australia is now the only state or territory yet to reach the mark.

Mr Morrison tweeted the Queensland and NT news on Thursday.

“Well done to Queensland and the NT! Confirming you’ve officially hit 80 per cent double dose vaccination, the target in our National Plan,” he wrote.

“Thank you to everyone who got their jab.

“Please get your booster if you’re due and help Australia continue to safely reopen and stay safely open.”

Tweet from @ScottMorrisonMP

Queensland hit the milestone five days before it reopens its borders to fully vaccinated travellers from domestic virus hotspots who test negative.

Nationally, more than 88 per cent of over-16s have been fully vaccinated while 93 per cent have had their first dose.

Meanwhile, a study has shown the Pfizer vaccine booster is an effective defence against the Omicron variant, despite the initial two doses being less effective.

Vaccine expert and chair of the Coalition for Epidemic Preparedness Jane Halton said Omicron would become the dominant variant in Australia and was here to stay.

“It’s arrived and we’re now seeing it spreading not just in Sydney but elsewhere,” Ms Halton told the Nine Network.

“The horse has bolted.”

The booster discussion came as Deputy Prime Minister Barnaby Joyce revealed he had tested positive to COVID-19 while on a work trip to Washington DC.

He said he had mild symptoms and had chosen to get tested.

All other members of the delegation travelling with Mr Joyce have s far tested negative.

There were 420 virus infections in NSW on Thursday, including eight Omicron cases, and one death.

Victoria had 1232 cases and nine fatalities, while the ACT had four infections.

-with AAP