On Wednesday morning, health authorities revealed a Sydney-based person who spent a weekend in Wellington tested positive for the virus on their return to NSW.

In response, Jacinda Ardern's cabinet held an emergency meeting, deciding against a lockdown, but to increase safety measures.

"We've had a positive case and that person had some extensive potential exposure in the Wellington region," COVID-19 Minister Chris Hipkins said.

The level two restrictions - which kicked in at 6pm on Wednesday and will last until Sunday - include mask-wearing in public, gathering caps and the encouragement of working from home.

Local transmission is yet to be identified: any new cases would snap NZ's streak of more than 100 days without community transmission.

Many see fresh cases and Wellington's first lockdown in a year as likely.

"That's too early to say," Ms Ardern, who has a planned trip to Australia in 10 days, said.

"We plan for a worst case scenario, be cautious in our approach as it's always better to plan that way than to have regrets later."

University of Otago epidemiologist Amanda Kvalsvig said Wellington "had two possible futures developing".

"If we're lucky, the COVID-19 positive visitor may not transmit the infection to anyone," she said.

"But if we're unlucky we may soon be dealing with a large outbreak."

In Wellington's favour is the fact the traveller has one dose of vaccine.

The government has suspended travel from NSW until Friday at the earliest - on the same day travel with Victoria has resumed after their outbreak.

The Sydney traveller arrived in Wellington on Friday night before heading back on Monday.

The most likely scenario - based on testing and the person's symptoms - is he caught the virus in Australia and travelled to NZ while contagious.

Four close contacts in NZ have already been identified: two people from Palmerston North and Tauranga, each of whom have returned a negative test.

Director General of Health Ashley Bloomfield said genomic testing will show whether the case is linked to the current Bondi outbreak.

"We'll get that information as quickly as possible from our Australian colleagues," he said.

"The key thing here is, whatever the variant, the response is the same."

Over a dozen locations of interest in Wellington have been identified, including national museum Te Papa, Jack Hackett's Irish Pub, and several cafes or restaurants.

A full list is on NZ's Ministry of Health website: anyone who visited those places at the same time has been asked to isolate and get tested.

The imported case is likely to heap pressure on the government's response to the NSW cases.

A community case in the Sydney suburb of Bondi was first identified last Wednesday.

NZ officials only made the decision to close the trans-Tasman bubble on Tuesday night, in response to a new batch of 10 cases.

Australian Associated Press