In bad news for overseas-based New Zealanders looking to come home after months stranded abroad, a call is unlikely before Christmas, as scientists assess how harmful the new COVID-19 variant is.

First detected in South Africa, Omicron has now spread to dozens of countries including Australia, which has reported 25 cases as of Monday afternoon.

That's of major concern to New Zealand, which is starting its reopening to the world with its trans-Tasman ally.

The slow and phased shift starts on January 17, when Australian-based Kiwis can bypass the compulsory hotel quarantine (MIQ) on arrival and self-isolate for a week.

New Zealander of the Year and microbiologist Siouxsie Wiles said the government should "absolutely" change tack if the new variant threatened Kiwis' ability to manage COVID-19.

"If Omicron ends up being the worst case scenario it could be, we would be foolish not to respond appropriately," she said.

"It would put everything we've done at risk.

"It's awful for those who want to see families overseas or those who want to come home, but we have to support our health system."

Early indications suggest the variant is more transmissible than Delta, but it's unclear whether it causes a greater rate of hospitalisation.

It's that data that will be essential to countries like New Zealand and Australia in considering border settings into 2022.

University of Auckland modeller Michael Plank said "a prudent approach at the border is sensible" while scientists worked it out.

"There's a lot we've yet to learn about Omicron. The key data is how well vaccines work. We'll know a lot more by mid-January," he told AAP.

University of Auckland virologist John Taylor said the evidence from South Africa was too thin to draw conclusions from.

"There is evidence in South Africa showing (infections) are rising faster than they did at the start of the Delta wave. The likelihood is it's more transmissible," he said.

"It's too early (to assess) the degree of protection from vaccination or previous infection.

"The third piece of evidence is the most encouraging. Although numbers of cases are increasing very steeply, the numbers of those cases requiring hospitalisation are not rising as steeply ... this variant may be less transmissible, it may be less virulent.

"We need to wait for non-theoretical evidence to come in. In the next couple of weeks, we should have hard data."

Ms Ardern - who wouldn't rule in or out border changes - said she was waiting keenly to read it.

"We are going to use the research and evidence that sits in front of us," she said.

"If we're presented with evidence that presents a significant danger to the population ... of course we will look at that."

New Zealand has vaccinated 88 per cent of Kiwis aged 12 and over, with 93 per cent receiving one dose.

University of Otago public health professor Michael Baker said "if we're serious about keeping Omicron out, we'll need to retain MIQ for a week".

"And that's a tough call. There are thousands of energised expats. There will be a lot of pressure to continue with that (reopening) plan ... that will be the toughest decision for the government: how to manage expectations at the border," he told AAP.

Australian Associated Press