The first 10 million doses of the double-shot jab will be delivered by the end of this year, destined for people under 50. Another 15 million booster shots are set to arrive next year, designed to guard against emerging COVID-19 strains.
Prime Minister Scott Morrison said it was important to prepare for possible variants of the disease.
"We're now well into the phase of dealing with what's coming next because the pandemic is not going anywhere," he said on Thursday.
Labor's health spokesman Mark Butler wants the government to explain why the deal has taken so long.
The US, Canada, the UK, European Union, Korea, Japan and Israel are already using the jab.
"Tens and tens of millions of doses of this state-of-the-art vaccine have already been delivered to the people in those countries," he told reporters in Canberra.
"Why do Australians have to wait until the end of this year?"
The vaccine and booster shot are still subject to approval from the Therapeutic Goods Administration before they can be used in Australia.
Health Minister Greg Hunt said the government had not changed its position on a no-fault compensation scheme for Moderna.
Mr Hunt said the company agreed to the government's terms.
Moderna is an mRNA - or messenger RNA - vaccine, which teaches cells how to make a protein to trigger an immune response.
Australia does not have the domestic capacity to manufacture such a jab. But the US biotech company has expressed interest in setting up an Australian base.
"We look forward to continuing discussions with Australia about establishing potential local manufacturing opportunities," Moderna chief executive Stephane Bancel said.
The government will commence an approach to market in coming days for mRNA vaccine manufacturing in Australia. But health department secretary Brendan Murphy said it was not likely to be established before next year.
Acting Victorian Premier James Merlino said his state was the prime candidate for the site, given it was home to 30,000 professionals working in the sector and had already put money on the table.
"We are the heart and soul of medical research in this country," Mr Merlino told reporters in Melbourne.
"We've also got funding through the breakthrough fund to provide additional support if required."
In the meantime, the federal government is focused on rolling out the Pfizer and AstraZeneca vaccines. More than 2.8 million vaccine doses have been administered across the country.