Health Minister Greg Hunt has also revealed cautious optimism the highly-infectious Omicron variant is milder than previous COVID-19 strains, and existing vaccines will prove effective against it.

The Therapeutic Goods Administration on Wednesday confirmed it had provisionally approved the vaccine for adults, provided they had received their second dose at least six months prior.

The Australian Technical Advisory Group on Immunisation was now considering the matter, and Mr Hunt was "very hopeful" of a "positive announcement" this week. "Australians will have two options for boosters very, very shortly," he said.

Moderna was the second vaccine approved for general use by the TGA as a booster in Australia, alongside Pfizer. "The TGA provisionally approved the booster dose following careful evaluation of the available data supporting safety and efficacy," its statement read.

"The TGA's decision was also informed by expert advice from the Advisory Committee on Vaccines, an independent committee with scientific, clinical and consumer representation.

"The Australian government continues to encourage individuals to complete a two-dose vaccination schedule as this enables strong protection against severe illness and hospitalisation."

And as Australia grappled with Omicron, Mr Hunt said the booster program was "fundamentally important" with more variants inevitable.

"We know that the boosters will help keep us safe. [It's] another addition to the booster program today, and we will await ATAGI ... [which is] meeting over the course of the coming days," he said.

Moderna and Pfizer were the two mRNA vaccines approved for use in Australia.

And amid concern the Omicron variant could undermine vaccine efficacy, Chief medical officer Paul Kelly has repeatedly stated the adaptable mRNA technology could prove vital.

Mutations in the variant could mean tweaks to existing vaccines were needed, but Professor Kelly was confident they would be able to produce adapted vaccines within 100 days.

Mr Hunt said, while it was likely Omicron was more transmissible, there was "emerging evidence" vaccines would prove effective.

"It's too early to make a final final judgment, but each day from around the world we have new hope that the vaccines will protect against serious illness, hospitalisation, loss of life from Omicron," he said.

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"And each day, we have new hope and evidence from around the world that this may be a milder form of the disease."

Immunocompromised Australians aged 12 and over will also be eligible for a Moderna booster 28 days after receiving their second dose.

Pfizer boosters have been offered to immunocompromised Australians since October, before the broader population was added to the rollout last month.

Roughly 15 million of the 25 million Moderna doses purchased by the federal government were slated for boosters in 2022.

Over 583,000 boosters had been administered across Australia by Wednesday afternoon. More than 93 per cent of Australians had received at least a first dose.

ATAGI was continuing to mull whether Pfizer will be granted final approval for five-to-12-year-olds. The Commonwealth expected a decision on that within days, and for that cohort to be offered a first dose by the first week of the 2022 school year.