Prime Minister Jacinda Ardern says if the country had not moved into alert level 4, daily case numbers could have been around 550.

Cabinet has confirmed all of New Zealand south of Auckland will move to level 3 from 11.59pm on Tuesday night. Prime Minister Jacinda Ardern says this will be for at least a week, to be reviewed at Cabinet next week.

Northland will likely join the rest of the country at alert level three from 11:59pm on Thursday, Ardern says.

Cabinet has also confirmed Auckland will remain at alert level four until 14 September. Cabinet will consider next steps for the region on 13 September.

Ardern says level 4 "is making a difference".

"The job is not yet done and we do need to keep going."

For Auckland and Northland, Ardern says the cases in Warkworth were found late in the lockdown and were not equivalent to the cases in Wellington, where cases were monitored and did not appear to have spread.

"We just haven't had that level of time for the cases we're concerned about in Warkworth, and with possible contacts beyond. Once we have that same level of reassurance in Northland we feel safe to move alert levels.

She says the government is awaiting test results from wastewater in Northland, and tests from people who were at locations of interest. If they all came back clear Northland could move to alert level 3 at 11.59 pm on Thursday.

"Just an indication here if all those tests come back clear," she says.

Ardern says if New Zealand had not moved into alert level 4, estimates of the number of new cases today could have been about 550.

"The more we do to limit our contact, the faster we will exit these restrictions," she says.

"Auckland is doing a huge service for all of us. And not just now, but throughout this pandemic. It's Auckland that has maintained our gateway to the world, that has done a lot of the heavy lifting in welcoming Kiwis home safely, that has worked hard to keep Kiwis safe when there has been an outbreak. Auckland has done it tough."

Ardern says the government is considering further restrictions under level 4 to prevent transmission occurring at the workplace. "It is a privilege to be open at level 4," she says.