Prime Minister Jacinda Ardern is defending the decision not to quarantine several thousand people coming into New Zealand from overseas who haven't shown any Covid-19 symptoms.

Yesterday expert epidemiologist Sir David Skegg urged the Government to quarantine all arrivals into New Zealand from overseas, saying that the global spread of the Covid-19 pandemic meant there was a progressively higher risk of an arrival carrying the deadly virus.

People carrying the disease may not show symptoms for days and several countries were taking precautionary measures by quarantining all overseas arrivals.

But Ardern said the current border restrictions, which came into effect just under a week ago, were working.

Every person is screened on arrival and symptomatic people are quarantined, while asymptomatic people are allowed to go home to self-isolate.

Those with no symptoms but without an adequate self-isolation plan were "essentially quarantined" in approved accommodation, she said.

The latest update from the Ministry of Health is that 115 people were in quarantine, and 1489 people were in approved accommodation.

A spokesman said about 30 per cent of arrivals had been put in approved accommodation, meaning about 3600 people have been allowed to head home to self-isolate.

Ardern said self-isolation has been working "successfully" since the beginning of February.

"We had over 10,000 people come back and self-isolate. The vast majority of New Zealanders are doing what they're asked to do."

She dismissed self-isolation being called a "high trust" model, a phrase used yesterday by Director General of Health Ashley Bloomfield.

"It's a high-trust environment. The vast majority of people understand their role and comply," Bloomfield told the Epidemic Response Committee yesterday.

"I have a lot of confidence in that. We rely on them to do that because we can't police every person."

Police would deal with those who didn't comply, Bloomfield told the committee.

Skegg had earlier told the committee about the risk of people arriving from overseas.

"Every effort must be made to prevent spread from New Zealanders returning from overseas. As each day goes by, the probability of those people carrying the virus increases."

He noted that Australia was putting all people arriving from overseas into quarantine, and he thought New Zealand should do the same.

But that quarantine had to be enforced and checked, as it is in Singapore by requiring those in quarantine to send text messages multiple times a day.

"I just don't think we have that level of checking going on," he told the committee.

He said many countries were using apps to track and trace people to ensure compliance, and Health Minister David Clark told the committee that the Government was in active conversation about the potential use of apps.

Today Ardern was dismissive of the idea.

"Other countries have used things like that," she said.

PM dismisses changing testing criteria earlier

Earlier today Director of Public Health Dr Caroline McElnay conceded that the testing regime did not show the true extent of community transmission.

About 1 per cent of the total of 708 cases were due to community transmission, she said, which is the key factor in determining whether the lockdown is working.

More testing in the next few weeks would provide a clearer picture of community transmission, she said.

• Covid19.govt.nz: The Government's official Covid-19 advisory website

Ardern said broader testing criteria would also lead to more testing.

Asked why the criteria wasn't widened weeks ago, she said it has always been based on expert advice.

"It's never been for me as a politician to determine who should be tested, but it's always been my message that we have built the capacity, so we have the ability for clinicians to test who they believe they need to test."

She said testing capacity had always exceeded the number of test requests, and clinicians have always had the ability to test at their discretion regardless of the criteria.