It comes after the Australian Technical Advisory Group on Immunisation conducted further investigations into the vaccine before providing the advice to the Federal Government this morning.
It's understood there are around 2.1 million people in that age bracket, who have not yet received the vaccine.
"We will have significant volumes of Pfizer coming in over the course of the coming weeks and months but we do ask for people's patience on this", he said.
The rate of VITT after second doses of AstraZeneca is thought to be lower than the rate for first doses, but NACI said the likelihood of developing this condition after getting a booster shot "has increased over time, with current estimates of approximately 1 per 600,000 people vaccinated".
"Anyone who has had a first dose of AstraZeneca without problem should feel very confident to have their second dose", Professor Kelly said.
Dr. Theresa Tam, Canada's chief public health officer, said that, based on early data, a mix-and-match regimen has "performed very well and seems to have a superior response" as it produces both a strong antibody and cellular response.
Three new confirmed and nine probable cases of TTS - likely linked to the Astra Zeneca vaccine - were recorded in Australia over the past week to Thursday, the Therapeutic Goods Administration said in their weekly update. "Cases overseas have been reported at all ages".
However, Australians who have already received AstraZeneca are being told to continue to get their second shot. The "mixed vaccine schedule" refers to the practice of using different products for the first and second doses.
There are 14 million doses of Pfizer-BioNTech and Moderna being delivered this week and next, including a donation of one million doses of Moderna's COVID-19 vaccine from the United States that arrived in Canada on Thursday.
More than 6.2 million doses have been administered nationwide since the rollout started in February.