University of Auckland epidemiologist Rod Jackson says New Zealanders should be very worried about someone with Omicron being in the wider community.
DJ Dimension - whose real name is Robert Etheridge - said in an Instagram post last night he tested positive after 10 days of isolation.
He said he was "devastated" to learn he had tested positive for the highly transmissible new variant while in the country.
The Ministry of Health has confirmed a border-related case of Omicron had briefly been active in the community on December 26 and 27.
Jackson said it was unusual the positive result came from a day nine test.
"The best possible explanation is it's a historical infection. That would be the best scenario that he doesn't actually have active Covid.
"The most likely and possible scenario is that he got it at MIQ which is a worry. If he brought it in, then it is a very long incubation period which would be highly unusual."
Jackson also added a warning: "We have to accept that even if this case didn't bring it in, it is going to come.
"I can't see how we can keep Omicron out.
"Our MIQ system has been working so far aside from the case that sparked the current outbreak, I don't see that we have had another breakthrough case since August.
"Omicron is just so contagious that I don't know if the MIQ system is capable of stopping it.
"I think that the longer we keep it out the better because the longer we have to learn how to deal with it.
"The Government should be very seriously considering requiring a rapid antigen test before people board a plane to NZ. I don't think a PCR within 72 hours works.
"The longer we keep it out the better. This should be a wake-up call to those who are still unvaccinated because whereas with Delta, we were pretty much certain that everyone was going to get Delta and it could've taken up to a year.
"With Omicron, everyone is going to get Omicron if they are not vaccinated - and it is going to come fast, which means there is no time to delay.
"So to people who are unvaccinated, Omicron should be another wake-up call.
"And also everyone who has been vaccinated for four months should be getting their booster.
"The evidence we have on Omicron is that you get far more antibody response if you have been boosted."
He said Omicron was "a double-edged sword".
"It looks like we're getting on top of Delta but the problem with Omicron is that it is so infectious even if it is milder, which it seems to be," Jackson said this morning.
"The impact of the virus is the combination of the contagiousness and the severity so you have to multiply one by the other.
"I don't want to downplay it but other than death which clearly is devastating [the problem is] overwhelming health services."
Etheridge arrived on a flight from the United Kingdom via Doha on December 16 fully vaccinated.
RNZ reports the ministry said the case had completed a full 10 days in isolation - seven days in a managed isolation and quarantine facility (MIQ) and three days in self-isolation, and had previously returned three negative tests while in MIQ.
"As required they were tested on day nine but did not wait for a negative test result before self-releasing," a spokesperson for the ministry said.
The day nine result was received on December 27 and the person was immediately transferred to an Auckland MIQ facility on the same day.
University of Auckland associate professor Siouxsie Wiles told RNZ New Zealanders needed to be prepared for Omicron.
"It's just really important that we don't think that seven days is okay and that people are still cautious ... After receiving several negative tests, people could still be incubating the virus and that's what it shows us."
She said people needed to play their part to minimise the risk of transmission.
"Wearing a mask when we're out and about even if we're vaccinated ... Being really cautious in places that have got bad ventilation. Getting tested if you have any symptoms - that's the really, really crucial bit."
Wiles said it was good that close contacts of the case had so far returned a negative result.
Etheridge visited bars and restaurants in central Auckland on Boxing Day.
He said he had unexpectedly returned the positive test two days after ending 10 days in isolation on arriving in New Zealand.
He had been due to perform at the Rhythm and Alps festival in Wānaka, but has now withdrawn from the event.
More details are expected to be given by the Covid Response Minister Chris Hipkins later today.
There are four casual locations of interest and one close - the Impala nightclub in Auckland's Shortland St between 11pm on December 26, and 3am on December 27.
Additional reporting RNZ