The mutant Covid-19 strain that sent large parts of England into a “tier 4" lockdown earlier this month has reached New Zealand.

There were 19 new cases of Covid-19 within New Zealand since the New Year. Of the new cases announced on Sunday, one was historical and the rest were in managed isolation.

However, six cases of the mutant UK strain have also been found in New Zealand, the Ministry of Health confirmed.

The six cases, five of whom travelled from the United Kingdom and one who travelled from South Africa, arrived into New Zealand between December 13 and 25.

Four of these cases have recovered and, after a final health check, were able to leave the facility.

The mutant strain is thought to be 70 per cent more transmissible than existing strains.

Experts in the UK have said there was no evidence that the new strain is more deadly, nor that it will be harder to vaccinate against.

Covid-19 Response Minister Chris Hipkins and Director-General of Health Dr Ashley Bloomfield addressed the media on Sunday.

The Covid-19 numbers are normally released via a press statement, however, the press conference was announced on Sunday as the Government introduced new rules for incoming travellers.

Travellers heading to New Zealand from the United Kingdom or the United States would need a negative test result for Covid-19 before entering the country.

The new rules would come into effect from January 15.

Covid-19 Response Minister Chris Hipkins said work was already underway to extend the requirement to other long haul flights to New Zealand.

“We intend the test to also have an added benefit of encouraging extra-cautious behaviour before people fly and during flights and layovers that will minimise exposure to the virus.”

Many countries have introduced tougher restrictions on travellers from the UK after a surge in people in that country testing positive for a Sars-CoV-2 virus variant.

Before the press conference, University of Otago epidemiologist Professor Michael Baker said he was not aware the new variant had been detected here but, given the number of arrivals from the UK, it was not surprising.

Baker said the new strain is another reason for New Zealand to introduce more rigorous controls before people enter the country.

He said he was relieved travelers from the UK and US need a negative Covid-19 test before leaving for New Zealand, but said stricter measures should be introduced for everyone arriving in a country from wherever the epidemic is out of control.

That would include agreement to isolate at home a week before departing and wear a mask through all transit, as well as getting a negative rapid test before departure.

"If we halve the number of infected people coming in here then we halve the risk to New Zealand."

Of the new cases announced on Sunday, the travellers who tested positive arrived from India, the UK, the US, Denmark, the Seychelles and South Africa.

The total number of active cases in New Zealand is 72, the total number of confirmed cases is 1,825.