Australia will not meet its ambitious target of completing COVID-19 vaccinations by October – and some people may have to wait until into 2022 to be fully covered, Health Department secretary Brendan Murphy has conceded.

Dr Murphy’s admission came as top government health officials told a Senate select committee that local and international supply issues with the AstraZeneca vaccine made it “impossible to predict” exactly when the first four million people in Australia would receive their first dose.

The Morrison government had initially expected that milestone by the end of March, with the rollout completed by October.

On Thursday, Dr Murphy said because AstraZeneca required two shots of the vaccine, it would be difficult to get it done by then.

“We don’t know whether we will be able to achieve two shots by the end of October,” he said.

“With a 12-week interval it will be difficult. I suspect it’s unlikely we will have completed the second doses of AstraZeneca vaccine by then.”

The admission came two days after Dr Murphy and Prime Minister Scott Morrison defended the federal government’s program.

More than 100,000 people in Australia have received their first dose of a coronavirus vaccine, but the figure is well short of what was promised with the rollout in its third week.

In January, Mr Morrison set the March target of four million. He later pushed that back to early April amid global supply pressure.

On Thursday, he said the speed of the rollout was “subject to events” – although he was still sticking to the October population-wide timeline.

“We will get this done by October as we said we would,” he told Seven.

“But we’ve got to do it safe – as Brendan Murphy said, this is not a race. What is most important is the health and safety of Australians in the vaccination program and that’s what we’ve principally focused on.

On Thursday, Health Department associate secretary Caroline Edwards said officials were “still planning and hoping” to finish by October.

“In the event that we didn’t get all shots by the end of October, the second shot would be finished six weeks after the end of October,” she said.

Because most Australians will receive the AstraZeneca jab, most will have to wait 12 weeks for their second shot. That means, if they might wait as long as January 2022 to be fully covered against the virus.

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Under questioning from Labor senator Katy Gallagher, Dr Murphy conceded the goal of four million by April was still questionable.

“It would be impossible to predict exactly when we will hit 4 million until we know what the CSL [local] production capability will be like, what the further international supplies will be like,” he said.

Australia’s supply of AstraZeneca vaccines was complicated when Italy blocked a shipment of 250,000 doses until European Union rules. The bulk of AstraZeneca vaccines for Australia will be produced by CSL in Melbourne, but the first of those will not be available until later in March.

Mr Morrison remained confident the vaccine rollout would gain speed.

“Towards the end of this month we’ll see the Australian-produced vaccines coming into the program,” he said on Thursday.

“That means we’ll be able to roll them out at around a million a week. When we get to that scale, I think we’ll see a real gear change.”

Deputy Labor leader Richard Marles said the opposition would hold the government to meeting its targets.

“Australians want to see the rollout obviously happen as safely as possible, as carefully as possible,” he told Sky News.

“But they also want to see it happen as quickly as possible, because that’s how we get through this.”

-with AAP