Morrison said the first batch of the additional doses would arrive on Saturday.
"The plane is on the tarmac now. It will be leaving tomorrow," he said.
The full arrival of the four million additional doses will occur over the coming weeks.
"This will see us double the Pfizer doses that we have during September," Morrison said.
He expressed gratitude to UK Prime Minister Boris Johnson for agreeing to send the extra doses to Australian shores.
"I want to thank (Johnson) for his personal commitment to this and his great friendship with Australia," Morrison said.
"There's been some very late-night discussions and negotiations and legal work taking place, especially over the course of the past week, to bring this to conclusion, but it has been a real commitment to Australia from Prime Minister Johnson."
The four million new doses will be distributed to states and territories on a per capita basis across GP clinics, pharmacies and state vaccination hubs.
"This will enable us to bring forward significantly the opportunity for Australia to open up again under the national plan," Morrison said.
Morrison said it was a good deal "between mates" due to Australia's urgent need for vaccines now, while the UK needed booster doses later.
"This is a good deal from Britain and a good deal for Australia. And it is a good deal because it makes the most of the doses that they have now, which we need," Morrison said.
"And the doses that we will have later that they will need.
"And so this supports their programme when it comes to boosters and other things of that nature, it supports our programme now."
