Prime Minister Scott Morrison, who arrived in the UK on Friday morning, thanked G7 host UK Prime Minister Boris Johnson "for bringing us together to put even more effort into this area because the virus doesn't know boundaries, the virus goes where it will".
He said Australia's contribution would feed into an effort led by Mr Johnson to vaccinate the world.
"These 20 million doses will go to support doses in our region, to ensure that we continue to exercise our responsibility as part of a broader global responsibility to combat this virus," Mr Morrison said.
World Health Organisation head Tedros Adhanom Ghebreyesus said earlier this week that 44 percent of all doses had been administered in rich countries but only 0.4 percent in the poorest.
"Sharing vaccines now is essential for ending the acute phase of the pandemic," Tedros said, urging G7 countries to do more to battle inequality in accessing coronavirus vaccines.
The G7 group of wealthy democracies - the UK, United States, Canada, France, Germany, Italy and Japan - have invited the leaders of Australia, South Korea, South Africa and India to take part in the three-day summit in Cornwall this year.
India's Prime Minister Narendra Modi will attend the G7 virtually due to the scale of the Covid-19 pandemic outbreak in his country.
Mr Morrison said Australia was in a strong position to support countries with vaccines because it had "supply contracts many times over what is needed for the Australian population" and that the jabs would make a difference.
"These aren't going in large warehouses which essentially (is) without going anywhere - we want to ensure that we are taking responsibility for our region, our family in our region," he said.
Mr Morrison's plane arrived at RAF Brize Norton in Oxfordshire on Friday morning after heavy fog cancelled plans for it to land at Cornwall's Newquay airport, requiring a drive of several hours for the prime minister to reach the summit venue in Carbis Bay, Cornwall, in England's southwest.
"This is the third occasion that we've had the privilege to be invited to be part of these discussions and there is a lot on this agenda for Australia," he said.
Mr Morrison was previously invited to the G7-plus 2019 summit in Biarritz, France, while the 2020 event was to be in the US but was cancelled due to the coronavirus pandemic.
"We've met on so many occasions over the past 18 months over screens," Mr Morrison said on Friday.
"There is no substitute for leaders getting together and doing what we are doing now - and there has never been a more important time to be doing that."
