Ms Ardern said she was encouraged by a fall in daily case numbers earlier this week, with 15 on Wednesday, 13 on Thursday and 11 on Friday.

However, the weekend has seen a jump: 23 cases were reported on Saturday and 20 on Sunday.

"I want to acknowledge how hard it is when numbers jump around as they have been in the last few days," she said.

"It's not a sign of failure. It's not a sign what we're doing isn't working. It just means Delta is harder.

"Our general direction is positive."

Ms Ardern downplayed reports of "mystery cases", saying advice from health officials was that "we still know where the vast majority of cases are coming from".

As of Sunday, health officials are yet to link 34 cases of the 922-strong outbreak.

Director-General of Health Ashley Bloomfield said "there is no widespread community transmission" but "we are assuming there may be other undetected cases" in the community.

"Yes, we have seen a few cases, which ... have taken us a bit of time to link," he said.

"There's been very targeted testing in communities not just symptomatic people but of asymptomatic people and its really important to us. The volume of testing is reassuring."

While cases have jumped, hospitalisations have fallen to half the peak rate: 18 Kiwis are currently in Auckland hospitals with COVID-19.

The cases come ahead of a review of COVID-19 alert levels on Monday.

While most of New Zealand has shifted from lockdown to alert level two, Auckland remains at level four, NZ's harshest restrictions.

Experts are predicting Auckland's lockdown will continue for at least another week, and Ms Ardern said she wouldn't entertain ending restrictions if it meant a Sydney-style explosion in cases.

"We do only need to look across at Australia at some of the alternative scenarios, and those alternative scenarios still involve lockdowns, but lockdowns with a large number of cases," she said.

"We all know why we have this goal of winning the battle against Delta ... that we can get back to a place where we have those freedoms again."

On Sunday, Ms Ardern also announced a second deal to increase New Zealand's vaccine stocks.

After banking an extra 250,000 doses of Pfizer from Spain, New Zealand has also agreed to buy 500,000 doses from Denmark, which ends any chance of New Zealand running out of vaccine this month.

"We now have the supply that we need to keep up what has been record-breaking rates of vaccination in New Zealand," Ms Ardern said.

"There is nothing holding us back."

Australian Associated Press