There are major fears one of New Zealand's chosen COVID-19 vaccines will fail to protect the country against the virus but the Government is waiting before they make a decision.

Scientists in Melbourne are working around the clock to produce vaccinations - one of the best shots the world has to end the pandemic. 

But while the Pfizer vaccine has been found to be 95 percent effective, the effectiveness of the AstraZeneca vaccine has fallen from 70 percent to 62 percent.  

"If they keep pushing the AstraZeneca vaccine they may not achieve herd immunity," Australian infectious disease professor Dr Michelle Anada-Rajah told Newshub.

Australian health experts are now calling for the roll-out of the AstraZeneca vaccine to be halted before it's even begun.

"[We should] try and pivot for the more efficacious vaccines because if we do that then at least we have a shot at herd immunity," Anada-Rajah said.

AstraZeneca is contracted to supply 2 billion doses to 25 countries - 7 million of those doses have been bought by New Zealand along with 10 million doses of the Novavax vaccine and 1.5 million doses of Pfizer's.

New Zealand's COVID-19 Response Minister Chris Hipkins said that their advice from the Ministry of Health's chief science advisor is that any call for a pause is premature.

He assured there was no cause for alarm or any change in New Zealand's plans.

"New Zealand's position remains the following: we have entered into an advance purchase agreement with AstraZeneca (AZ) to supply 7.6 million doses, which involves open sharing of all data about efficacy and safety which will be evaluated by Medsafe.

"The AZ and Oxford science team are making their data available as it comes to hand, and there will be more phase 3 data available in the coming weeks."

Hipkins said the deployment of the AstraZeneca vaccine overseas will help us to understand its effectiveness.

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"New Zealand has secured access to the four COVID-19 vaccines. Once each vaccine has gone through Medsafe's approval process, there will be a separate 'decision to use' step undertaken by Government before any vaccine is rolled out for public use."

Peter McIntyre from Otago University agreed there is "absolutely no justification in saying that it's second-rate or less desirable".

"It may turn out that when we have more information that it seems better in some respects and not as good in others," he said.

AstraZeneca is also the cheapest option of the available vaccines at $2 - $3 a shot, and it's also easier to transport, meaning it reaches more people.   

"It doesn't need to be stored at -70C," Prof Des Gorman from Auckland University said. "In fact, it doesn't need to be in a fridge."

Prof Gorman said Kiwi health officials won't want to muck up the rapid rollout of a mass COVID-19 vaccine programme. 

"We failed in the measles epidemic, we failed to get the influenza vaccine adequately distributed during the first COVID lockdown. So we clearly have logistic problems and now we're confronted with three different vaccines - one needs to be stored at -70C, one needs to be stored in a fridge and one doesn't."

And while the new, more contagious variant of COVID-19 has already arrived at New Zealand's border, officials aren't being too hasty.

It will be months before the first Kiwi gets a shot of a vaccine.