The luxury Auckland Pullman Hotel is now at the centre of an outbreak that has seen four people test positive with a contagious South African Covid strain this month - and three infected returnees leave undetected.

The hotel won't be accepting any new returnees and stays for those already in managed isolation there are being extended.

Officials are scrambling to contact and test nearly five dozen people who left isolation at the five-star hotel in the heart of Auckland's central business district over a two week period who may be harbouring and spreading the Covid infection in their communities.

This morning Covid Response Minister Chris Hipkins said the infection spread was alarming and investigations were under way to determine what had led to the current outbreak.

"There does appear to be something at the Pullman Hotel and we're looking very closely to identify what it was there," he told Newstalk ZB's Mike Hosking.

Those already in managed isolation at the hotel would be forced to stay there until they had contained the risk and the hotel would not be accepting new arrivals.

"The Pullman hotel will empty out a little bit and we're not letting anyone leave the Pullman hotel at this point until we're absolutely certain we've contained whatever the risk is," he said.

Last night the director general of health, Dr Ashley Bloomfield, said a number of measures were being imposed as a result of the infection breach.

It included a deep clean of commonly used areas, tighter restrictions on movement of returnees including no arrivals or departures from the facility, and increasing hotel ventilation.

The Ministry of Health was also requesting returnees who had recently left, to not fly, to stay home and have an additional test within 48 hours.

Staff posted at the facility were being restricted from working at other sites.

The ministry said 212 staff at the hotel had been tested, along with 192 guests currently in the facility.

Hipkins said of the 354 people who had left the managed isolation facility between January 9-24, 300 had returned negative tests.

On Sunday it was revealed a Northland woman had contracted Covid-19 before leaving her 14-day spell at the Pullman hotel on January 13.

It was found she contracted the same contagious South African variant near the end of her time in managed isolation from a fellow returnee staying across the corridor.

That first infected traveller was transferred to Jet Park quarantine facility on January 13 after Covid-19 was detected during routine testing.

The infected woman isolated, along with her husband, at her home south of Whangarei.

She has since recovered but not before sparking a region-wide health scare after she visited 32 locations over nine days.

Now two more - a child and an adult - who were also staying at the Pullman hotel and left on January 15 have tested positive.

A third person in the family's bubble has tested negative.

Last night the infected pair were moved to Auckland's quarantine facility near the airport.

Earlier health officials described the pair as asymptomatic.

This morning Hipkins revealed the pair's infection was linked to the same genome as the Northland case.