Despite case number growing more slowly, Prime Minister Jacinda Ardern says it is still too early to tell if New Zealand's measures have been effective.

Healthcare workers say coronavirus tests are being withheld because of limited supply, despite the prime minister's insistence clinicians have both the resources and permission to test.

A new coronavirus testing criteria was released late on Wednesday, permitting the testing of patients with Covid-19 symptoms but no connection to overseas travel or another coronavirus case.

Prime Minister Jacinda Ardern, who announced the expansion of the criteria the day prior, again said there was ample testing capacity at a Wednesday press conference — it just hadn't been used by clinicians.

Hagen Hopkins/Getty Images Prime Minister Jacinda Ardern speaks to media during a press conference at Parliament on March 31, in Wellington.

But a Wellington GP, who spoke to Stuff on the condition of anonymity, said there were not enough testing swabs for the number of patients presenting Covid-19 symptoms. His clinic had ordered 30, but received five as the laboratory was trying to preserve supplies.

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"On the one hand they say we have the ability to test 3000 people a day and, 'test, test, test'. On the other hand we only get given five swabs when we order some because they're running out."

KEVIN STENT/STUFF A drive-through testing for coronavirus at Newtown School in Wellington.

And some swabs of patients were not run by the laboratory for failing to strictly meet the prior testing criteria.

Staff from two Wellington clinics told Stuff on Wednesday morning they had not received any information from the ministry regarding the expanding criteria, and their clinics would go ahead and test people with possible Covid-19 symptoms until further guidance was issued.

But at a Canterbury GP clinic, a nurse said her practice had been told to continue using the former, stricter criteria in the absence of an update from the ministry.

The nurse, who similarly spoke on the condition of anonymity, said the ministry had told clinical leaders there were not enough swabs and the labs did not have enough reagents to test everyone with symptoms.

"We are seeing many people with coronavirus symptoms who we cannot test."

She said GPs at her practice had used their discretion - as the prime minister and director general of health Ashley Bloomfield had repeatedly recommended - and swabbed people with symptoms, only to have the labs refuse to test them.

Meaning health workers and officials had no idea how many coronavirus cases were out there — "We will probably never know due to the lack of swabs".

Ardern said a distribution issue was to blame for GP clinics reporting a lack of swabs and, as she understood it, laboratories were not rejecting swabs.

"I can't remember one day, off the top of my head, where the tests have met the capacity that we've had ... We have the swabs, we're constantly needing to make sure that they are across across the country.

"What I have seen are reports of where GPs have referred people to those testing clinics, it's something that I have raised with the Director-General or Health, can we get a handle of that inconsistent messaging, because that's frustrating."

Facing questions about the failure to fully use testing capacity, Ardern said clinicians had for weeks been permitted to test on the basis of "clinical concern", if a patient did not meet the testing criteria.

"You will have heard me constantly and consistently saying that clinicians needed to use their discretion," she said.

There were 2093 tests were run on Tuesday. Ardern acknowledged the number of tests run has fallen in recent days, despite the capacity rising to 3700 a day.

But a Ministry of Health spokesman said the daily testing numbers would no longer be provided, in favour of an seven-day average — currently at 1843 tests.

Dr Samantha Murton, president of the Royal College of GPs, said the number of swabs provided to each District Health Board differed, and a national distribution system may be required.

"In some practices, they've got enough swabs, they can get their PPE, it's all running smoothly. And in other practices, you're limited the number of swabs they're allowed," she said.

University of Otago Associate Professor of microbiology and immunology Dr James Ussher said his laboratory - Dunedin's Southern Community Laboratories - had been rejecting a fair number of testing swabs for their failure to meet the criteria for testing.

This was not an unusual practice for a laboratory, he said. But they have more recently begun running all swabs received, as there was more community transmission occurring.

The Ministry of Health appeared to have enough swabs and chemical reagents to run the tests, he said. The Government had been working hard to provide certainty around the supply chain.

Ussher said broadening the test criteria was like "trying to find the needles in the haystack" as anyone who had not been overseas or in contact with a confirmed case was at very low risk of having the virus, but this was needed to eradicate coronavirus.