A Covid-19 modeller warns the red traffic light would do little to stop the rampant spread of Omicron in New Zealand, after case numbers quadrupled in one week in New South Wales.

Pressure is mounting on the government for further limits on incoming travellers, despite pleas from people stuck overseas.

University of Canterbury professor Michael Plank says New South Wales has a higher vaccination rate than Aotearoa at 93 per cent of the eligible population double-dosed, so its Omicron outbreak could be mirrored here.

The state's daily case numbers have gone up four-fold in seven days with 3763 new cases reported on Wednesday and a total of 203 people in hospital.

Australian health authorities blame the new variant for most of the recent infections.

Professor Michael Plank. Photo: Supplied.

"If we just get one case it is very variable as to whether that case perhaps passes it on. So if we pick it up early, it's possible we could still stamp it out. But if Omicron became established in the community I expect it would spread very very quickly at a similar speed to what we've seen in New South Wales," Plank says.

"Watching New South Wales over the next few weeks will give us a really good guide as to what we could expect if Omicron came here. There's early evidence Omicron is a bit milder and the hospitalisation rate is a little bit lower than Delta ... but there's still a risk that the sheer number of people being infected would put significant pressure on our healthcare systems."

Plank says the traffic light system relies on vaccinations and early data shows vaccinations are less effective against Omicron, although they reduced the chance of hospitalisation.

He believes mask wearing and venue capacity limits will only slow the spread slightly.

On December 22 the number of people in MIQ with Omicron was 28.

To stop the variant getting out, the government has lengthened MIQ stays for arrivals, delayed quarantine-free travel from Australia until at least late February, and shortened the time for pre-departure tests.

Plank also wants rapid tests for returnees right before they flew, and he backs calls from epidemiologist Michael Baker for the number of new arrivals to be slashed.

Nurses Society director David Wills agrees.

"Given the way Omicron has spread you have to regard every country as a potential threat. That's why reducing numbers is really important. There may well be further tightening in the not-too-distant future and if there is, we'd welcome that," he says.

It was likely there would be strong pushback from people overseas fighting to freely fly home.

Among them was Sally Birch's mother Kathy, who was in her 70s and has been stuck in Brisbane for six months. She plans to return on January 19 to care for her 96-year-old mother.

Those plans have been shattered.

"Nana's not getting younger. Mum's not getting any younger, Christmas is coming and about to go and it's pretty tough," she says.

Already about 1700 fewer MIQ rooms are in use compared to late October.

There are 2955 recent arrivals in the hotels.

MIQ data shows occupancy has fluctuated week to week.

Officials say the changes are due to cohorting, cleaning, maintenance and flight changes, not in response to Omicron, or any more contingency rooms set aside.

-RNZ/Katie Todd.