Officials are investigating if a returnee in managed isolation who tested positive for Covid-19 at Auckland's Grand Mercure hotel caught the virus from another MIQ guest.
And 23 other returnees who shared a bus with the positive case to an Auckland exercise area have now had their MIQ stays extended by five days as a precaution.
A Ministry of Health spokesperson said the person who tested positive travelled from the United Kingdom via Singapore and returned a positive day 12 test.
They had travelled twice by bus to an exercise area in Mt Albert, once on March 19 and once one March 21.
"All 23 people who shared a bus ride with this person have been asked to stay until five days after their exposure. They will also be tested again."
The guests have been confined to their rooms.
A letter from Auckland District Health Board medical officer of health Mariam Parwaiz to one of the affected guests said they were required t stay in MIQ for another five days and were considered a "casual plus" contact, Stuff reported.
A Covid-19 swab would be taken on March 26.
"Your situation will be reconsidered on March 27," the letter said.
"You have the right to instruct and consult a lawyer in accordance with the New Zealand Bill of Rights Act 1990."
Meanwhile, Auckland Regional Public Health Service is investigating whether the positive case was infected by another MIQ guest.
The spokesperson said the "standard" source investigation was done for all cases in MIQ.
"The service is waiting for whole-genome sequencing for any other possible links to other cases in the MIQ.
"Returnees are routinely tested on day 12 as the infection can take longer to develop in some people."
An MIQ spokesman said the Grand Mercure was one of the Auckland facilities where returnees were bused to a secure exercise area because there was no suitable exercise area at the hotel.
"Being able to exercise supports returnees' health and wellbeing while they are in managed isolation."
Though busing MIQ returnees to exercise areas outside their hotels has been criticised in recent weeks as risky, there were no plans to amend the practice, the spokesperson said.
"However, the Ministry of Health and Managed Isolation and Quarantine continue to regularly review our policies and procedures to ensure they remain fit for purpose.
"MIQ is undertaking an internal investigation to determine what happened in this case and what improvements can be made to strengthen our processes.
"We understand this disruption to returnees' plans will be distressing. However, the safety of all returnees, our staff and the wider community remains our top priority."
More than 124,000 returnees have been through the MIQ system to date.