Australia's already struggling rollout was dented in July, when the Australian Technical Advisory Group on Immunisation advised AstraZeneca was no longer preferred for the under-60 age range.

Prime Minister Scott Morrison a week later claimed he had made "constant appeals" to ATAGI to change its advice.

Co-chair Christopher Blyth on Tuesday told the Senate committee on COVID-19 that ATAGI had always stressed its advice could change during large outbreaks, as it had in NSW and Victoria. He said the group was blindsided by the Prime Minister's public rebuke.

"I was shocked and surprised. I suppose I was surprised that given the context that we were in, that that advice was clearly levelled at ATAGI. I did not expect that," he said.

"Members I spoke to were [also] taken aback by those comments, because clearly it's much more complex than just an individual group of technical advisors.

"But we need to stick to our swim lanes and continue to provide advice. That's what we do."

Dr Blyth said he was not aware of any formal or informal correspondence from the Prime Minister since the comments.

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But Health Department Secretary Brendan Murphy, also appearing before the committee, stressed the government had always accepted ATAGI's recommendations.

He said he had reached out to ATAGI to reiterate they enjoyed the government's full support.

"I think the Prime Minister was, as were we all, probably frustrated by the thrombosis issue that happened with AstraZeneca. That was going to be our workhorse vaccine," he said.

"The ATAGI advice, which was based on the epidemiology at the time, did undoubtedly lead to a significant slowing in the rollout."

Mr Morrison joined Health Minister Greg Hunt and Professor Murphy for a snap, late-night press conference less than an hour after they received the advice.

Professor Murphy said Mr Morrison and Mr Hunt were presented with ATAGI's advice at the same time.

"They ultimately they made the decision to make the programmatic changes, consistent with that ATAGI advice," Professor Murphy said.

But Labor senator Katy Gallagher insisted Mr Morrison had "openly and publicly blamed" ATAGI for delays to the rollout, which were exacerbated by a lack of mRNA vaccines.

"It's a pretty big thing for a Prime Minister to come out on a doorstop and basically say: it's the technical advisors responsibility that we are where we are," she said.

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