Dr Ashley Bloomfield says the "stamp it out" phase for Covid-19 will be in place as long as is necessary.

Health Director-General Ashley Bloomfield says the Government will keep the country or parts of it in lockdown for as long as possible to "stamp out" coronavirus.

Asked if there was a backup plan if lockdown failed to stop mass transmission of the deadly virus, Bloomfield said the plan was to avoid that situation as the number of deaths would be untenable.

"Our plan is not to get into that situation, and that's why we are taking such significant measures," Bloomfield said.

"One of our key drivers on this has been to protect our healthcare system, protect our population – keep them well – and to not see the number of deaths modelling suggested would happen. We didn't see that as tenable."

HAGEN HOPKINS/GETTY IMAGES Director-General of Health Dr Ashley Bloomfield said the country would remain in lockdown for as long as necessary.

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The death toll in the modelling used by the Government runs as high as 27,600 if suppression efforts fail.

Asked again if there was any backup plan, Bloomfield said the Government was "in the plan" and that could see lockdown maintained for a long while.

"Well we are in the plan. The plan is the plan, isn't it. That's why we've got alert level 4. If we need to we will keep the measures in place until we see that dropoff in cases. We will then maintain that long-term stamp-it-out phase."

He referred back to the pandemic influenza plan which has acted as a blueprint for government action, but said the Government had essentially decided to stay away from the phase of that plan where the disease was "managed".

"We decided we didn't want to move to manage it. We wanted to stay in an extended keep it out, stamp it out phase. And the stamp it out we are trying to do at the moment is stamp it out altogether."

Prime Minister Jacinda Ardern said the global evidence was that self-isolation when properly applied worked.

"I haven't seen anywhere where a country has opted into these kinds of options, as New Zealand has, where we haven't seen an impact on transmission," Ardern said.

She said the four-step alert system built in other methods of slowing transmission other than full lockdown however.

"For instance the ongoing public health campaign: the basics of making sure that we are all washing our hands, that we are coughing and sneezing into our elbows, all of those need to be practiced regardless."

She said there would be "indicators" of success for the lockdown but these were unlikely to be hard and fast numerical targets.

"At level 4 what we are looking at is signs that we have transmission back under control. We will be building in thinking about the number of cases we have, the level of community transmission, what's happening with our clusters."

"We will also be looking at regions - do we have outbreaks in certain regions that aren't in control? Level 4 is also back about wresting back control."

University of Otago epidemiologist Nick Wilson, who has worked on the models for the Government, said on Tuesday that the Government needed clarity around a Plan A and a Plan B if it failed.

This might involve all older people staying in lockdown but other age groups restarting something closer to normal life.

WHAT WILL LET LOCKDOWN LIFT?

Bloomfield said the Government was currently working out what exactly the conditions would need to be for either the whole country or parts of it to exit the level 4 lockdown.

He expected this work would be completed in the next few days.

There were 89 new probable or confirmed cases of Covid-19 revealed on Thursday.

Bloomfield said the country was yet to "flatten the curve" on the virus.

He said 2563 tests for Covid-19 were run on Wednesday, the greatest number of tests completed so far. This is more than 1000 tests short of the daily testing capacity of 3700.

The total number of cases run now exceeded 26,000, and Bloomfield said New Zealand had conducted a good amount of tests when compared to other countries at this stage of their response to the spread of Covid-19.

Police commissioner Mike Bush, also fronting the Thursday afternoon update, said there were at least four arrests, possibly five, of people who were breaching the lockdown.