New Zealand will remain in lockdown until at least midnight on Friday, August 27.

Prime Minister Jacinda Ardern this afternoon announced Cabinet's decision to extend the nationwide Alert Level 4. It had been due to finish at 11.59pm tomorrow.

Auckland, the centre of the outbreak, will remain in lockdown until Tuesday, August 31, at least.

There are 107 community cases in this outbreak, all likely to be the Delta variant, including 35 announced today.

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The vast majority are in Auckland, along with eight in Wellington.

So far, there have been no cases elsewhere in the country and no evidence from wastewater testing that the virus has spread to the South Island.

But Ardern says the 48-hour travel window means "we have in essence only had 72 hours . . . where everyone across the country has stayed put".

During that window, 8000 people flew out of Auckland.

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Modelling suggests the outbreak, now on day 6, has not yet reached its peak.

"That does mean though that the safest option for all of us right now is to hold the course for longer," Ardern said.

"This will allow us to have additional data and assurances to whether the virus has spread from our existing sites and how contained it's likely to be."

There are now 320 locations of interest, and more than 13,000 close contacts.

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Ardern showed a heat map showing the location of known contacts, appearing to show some in Invercargill, Dunedin, Queenstown and Wanaka.

The nationwide decision will be reviewed on Friday, while Auckland will be reviewed on Monday.

More follows.