Warnings New Zealanders face the possibility of spending Christmas in lockdown are being described as "doom and gloom" and not helpful by a leading medical professor.

There are now 511 cases of COVID-19 in the community since an outbreak of the highly infectious Delta variant unfolded about a fortnight ago.

Auckland, the epicentre of the outbreak, and Northland are set to spend weeks in COVID-19 alert level 4 - having already spent a fortnight under those restrictions and at least another two weeks looking all but certain.

But the rest of New Zealand is preparing to move to alert level 3 at 11:59pm on Tuesday. There are no community cases of COVID-19 anywhere outside of Auckland, other than 15 in Wellington.

Despite this, experts have urged caution.

"The last thing we want is to move down the alert levels too quickly so that in a few weeks' time we end up with cases we didn't know about... and basically Delta comes back and we end up having to go back to alert level 4," University of Auckland microbiologist Siouxsie Wiles said on Saturday. "The timing of this means if that happens, we'd probably be in restrictions at Christmas time," Dr Wiles added.

But University of Auckland professor of medicine Des Gorman said "doom and gloom predictions" weren't helpful.

"I think it's just disincentivising people and it's going to discourage the very behaviours that we need," he told The AM Show. "I don't think we should… look at this with rose-tinted spectacles but, having said that, let's not overegg the souffle for goodness sake. If we're in lockdown on Christmas, the only reason for that; we would have had a New South Wales situation where people have said, 'I'm not going to sit at home, I'm going to wander around the neighbourhood.'"

New South Wales, the Australian state home to Sydney, is grappling with an outbreak of the Delta variant despite being in a lockdown for nearly two months. 

Many are refusing to comply with restrictions. On Sunday, one COVID-positive man was arrested for not self-isolating - the fourth time he had been caught leaving his home while infectious.

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The state reported a record 1218 community COVID-19 infections on Sunday, exceeding the previous day's record of 1035. 

On the contrary, Dr Gorman said New Zealand's alert level 4 lockdown was "almost certainly" working.

"If you look at the numbers, it's entirely consistent with spread within households and pre-lockdown infection. I can't see any evidence here at all that there's something dire or very worrying going on.

"I don't think there's anything in the data to suggest this lockdown's not working."

But Dr Gorman said Delta's spread did catch the Government and hoped lessons would be learned.

"People got tested late, it took a day or so for the results to come back and people have been left in their homes for another day or so before they have been shifted on to a quarantine facility. It's made sure if there is one person who is infected in a family of six, by the time this has all occurred all six of you are infected.

"There's nothing new that we are seeing here. We've known about this strain for a long, long time."