New Zealand has recorded just 29 new cases of Covid-19 in the past 24 hours, but the prime minister says a decision on changing the alert level will only be made two days before the lockdown ends.

It is the fourth day in a row there has been a day-on-day drop.

Director-General of Health Dr Ashley Bloomfield said there are 23 new confirmed cases of Covid-19 and six probable cases in New Zealand today. That brings the total number of cases to 1239.

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There have been no additional deaths, but 14 people are in hospital, with four in ICU at North Shore, Middlemore and Dunedin hospitals.

Dr Bloomfield said 317 people have now recovered from the coronavirus, with 35 recovering in the past day.

He said 41 percent of cases are travel-related, 44 are related to close contacts and 2 percent are community transmission. Thirteen percent are still under investigation.

There are still 12 clusters, the three largest are the wedding in Bluff with 87 cases, Marist College with 84 and Matamata with 66.

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"Our ability to contain those clusters has been greatly enhanced because we are in the alert level 4 lockdown situation, so we are confident now that we are on top of those clusters and that any additional cases are largely within the bubbles within each of those clusters."

The daily numbers of new cases of Covid-19 have been slowly dropping this week. There were 50 new probable and confirmed cases reported yesterday, with 54 on Tuesday and 67 on Monday.

Dr Bloomfield said he expected case numbers to stay low, with a few bumps up and down.

Despite the drop in new cases, Prime Minister Jacinda Ardern made it clear the lockdown will not end before the four-week period is up.

She said any decision on changing the country's alert level will be made on 20 April, two days before the four-week lockdown ends.

She said this meant businesses will have two days to prepare for any move into alert level 3.

But Ardern said the country's response to the lockdown over the past two weeks had been "huge".

"In the face of the greatest threat to human health that we have faced in over a century, Kiwis have quietly and collectively implemented a nationwide wall of defence.

"You are breaking the chain of transmission and you did it for each other," she said.

Ardern said initial modelling showed New Zealand was on a similar trajectory to Italy and Spain, but current modelling showed the lockdown measures were working.

"As we head into Easter, I say thank you to you and your bubble," Ardern said.