A top health expert is calling on the Government to stop people coming to New Zealand from the US and UK as new COVID-19 variants continue to spread amongst their populations.

Both the South African and the UK variants have been identified as between 50 and 70 percent more contagious than the strain that previously circulated New Zealand.

On Sunday the Ministry of Health revealed the South African variant had been found in a case at our border for the first time.

Epidemiologist Dr Michael Baker is concerned the increase in transmissibility makes it far easier for the virus to make it through our barriers.

"This is very dangerous period for New Zealand. We'll have to wait months before we have enough vaccines to protect people in this country and before there's enough vaccine coverage overseas to turn down the pandemic. At the moment it's just becoming more intense in most countries where we're receiving travellers from and that's a huge increase in risk," he tells Newshub.

Dr Baker wants the Government to take "very assertive action to protect New Zealand".

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"I really think we need to be turning down the flow of these infected people from these countries, unfortunately," he says.

"At one extreme I think we should consider what other countries are doing now and that is actually suspending travel from countries, the UK and the US, for a period until we are confident we can manage this risk.

"The alternative is we work much harder to prevent infected people from getting on flights in the first place."

Dr Baker warns that if the virus does manage to escape from out of our Managed Isolation and Quarantine systems, it would be a disaster.

"It would be a catastrophe for New Zealand because it would mean going back into lockdown, potentially quite a severe lockdown - maybe level 3 or 4 - for a prolonged period," he tells Newshub.

"Unfortunately it's [the Government] going to have to use its current systems, the alert level system and all the other controls we've become very familiar with.

"It will just have to use them in a much more intensive way and for much longer."