Covid-19 experts are nervously observing an ever-increasing number of cases at the border, as the threat of an Omicron outbreak looms.

The highly transmissible variant has rapidly spread around the globe and New Zealand has dodged a community outbreak so far.

But with the escalating number of overseas returnees testing positive, there are fears a new wave of the virus could be out in the community within weeks.

Epidemiologist and University of Otago professor Michael Baker calls the variant a "huge threat" and said it was not a matter of if there was an outbreak, but when.

Baker was concerned there may have been undetected transmission of the virus - whether that was the Delta or Omicron variant - during the Christmas and New Year period.

"It will take a while for people to people to develop symptoms if they were exposed. Everyone should be aware of getting any cold or flu symptoms, which is unusual for this time of year."

A Managed Isolation and Quarantine spokesperson told RNZ the hotels are well set up to cater for Omicron cases and a number of precautionary measures were in place to manage the risk.

This included travellers staying 10 days in MIQ and undergoing four tests during that time.

Anyone who tested positive is treated as an Omicron case until proven otherwise by genome sequencing.

Despite these measures, Baker is doubtful the country can make it through the month without the Omicron variant escaping.

"We're getting more than 20 cases a day in the last three days. That's going to put huge strain on the MIQ system, as we know every infected that arrives increases the risk of border failure."

There were 64 new border-related cases in MIQ during the weekend, bringing the total to 227.

University of Otago senior lecturer Lesley Gray says this does not bode well.

"We know that for every approximately 100 that we have in MIQ there is a risk that there might be one that might end up in the community."

From 7 January, travellers to New Zealand must return a negative test within 48 hours of their departure, down from 72 hours.

Director of Public Health Dr Caroline McElnay has previously said that people may have been incubating Covid-19 before their flight or been exposed during their travel.

Lesley Gray is concerned people are catching the virus within that short-time frame.

"We do have to ask the questions of 'how, what, when, and why'. As these people travel, they're distanced for the most part on the planes, when they're in airports they're wearing masks and they have to take a reasonable number of precautions."

She urges Kiwis to ask themselves if they are ready for an Omicron outbreak.

This includes having adequate supplies and a suitable place to quarantine if needed.

She says getting a booster shot, scanning in, mask-wearing, and testing are among the best tools to tackle Omicron.

-RNZ/Jean Bell.