Stricter measures than for past outbreaks could be on the cards if the Australian visitor turns out to have had the "explosive" Delta variant of Covid-19, a top scientist warns.
From 6pm the Wellington region, including Wairarapa and Kapiti Coast, moved to alert level 2 until 11.59pm on Sunday.
The news has led to thousands seeking Covid-19 tests and Healthline running hot.
The change in alert levels comes after a Sydney man travelled to the region over the weekend before returning a positive Covid-19 result on his return to Australia.
The traveller works in a healthcare workplace near Bondi Junction - an area linked to the current outbreak in Sydney.
It is feared he could have the Delta variant, which is now at the centre of the outbreak.
Covid-19 Response Minister Chris Hipkins said the chances of an outbreak in New Zealand remained "low-risk". Four of the traveller's close contacts here had already returned negative results.
The traveller had received one AstraZeneca vaccine jab. Hipkins did not know if the traveller's partner had also contracted Covid.
Professor Michael Plank, a modeller at Te Punaha Matatini and Canterbury University, said if the variant turned out to be Delta there could be further cause for concern.
"We know that it's a lot more transmissible – in fact, it's about twice as transmissible as the original variants of Covid-19."
It can be transmitted by fleeting encounters, and has led to a surge in cases in the UK despite 82 per cent of adults having received one vaccine dose, and 60 per cent being fully vaccinated.
The variant also leads to 1.85 more hospitalisations, and vaccines are less effective against it, especially with only one dose.
"If it does take hold it can spread really explosively," Plank said.
Within months of its detection, the Delta variant has become what one US public health scientist has called the "greatest threat" of the pandemic in 2021.
Perhaps the biggest risk factor was its ability to spread quickly through populations: it was thought to be partly responsible for India's disastrous second wave this year.
Given this risk, Plank said the decision to move Wellington to alert level 2 was "sensible", and would give contact tracers time to get ahead of the virus.
But if community transmission of Delta eventuated and it turned out the traveller had visited more crowded places and cases emerged, Plank said quick action to ramp up alert levels was needed.
A key variable would be the type of cases. If they were close contacts it was of less concern, but if they were simply in the same bar or cafe it could signal potential for many more cases.
Plank said the alert levels themselves could also be reviewed, with measures like different gathering sizes and mask use in crowded locations introduced.
The Pfizer vaccine was still effective against Delta, but that was less relevant given New Zealand's low vaccination rate, Planks said.
Hipkins told Newstalk ZB the risk of an outbreak was low but there's "not no risk".
"We know we had someone with Covid-19 in an environment where it could potentially spread reasonably quickly - a crowded pub, an exhibition at the museum attended by a large number of people.
"But overall it's low risk at this point."
Hipkins said the variant is expected to be known tomorrow morning.
The Government has faced pressure over its response times over the latest case.
In Parliament, under questioning from National's Covid-19 spokesman Chris Bishop, Hipkins said he became aware of the case after 8pm on Tuesday.
He was not aware of the case when the decision was made to pause quarantine-free travel with New South Wales. That decision was made between 5pm and 6pm, although it was about 8.30pm.
Hipkins defended the time it took for the Ministry to publish the locations of interest.
Despite becoming aware of several places on Tuesday evening, they were not published until this morning.
So far over a dozen places of interest have been identified.
Hipkins said part of the delay was due to the verification process, and fact it involved talking to people on the other side of the Tasman in the middle of the night.
At level 2 there will be limits on gathering sizes and no more than 100 people could now meet at one time.
Face masks were now compulsory on public transport and people were encouraged to wear them while waiting for transport.
Hipkins also encouraged people to wear masks where social distancing was not possible.
Schools would stay open, as would ECE centres.
Hospitality outlets could open but needed to apply the three S's - seated, separate, and having a single server.
Contact tracers were working "at pace", Hipkins said.
Hipkins said the situation was a reminder of the need to use the Covid tracer app and to have the Bluetooth function turned on.
If people had been in the Wellington region over the weekend, they needed to take the alert level restrictions with them.
Today, Bloomfield also revealed New Zealand had administered 1 million doses of the Pfizer vaccine. DHBs were 7.5 per cent ahead of plan, he said.