An expert says New Zealand should consider reviewing how long incoming travellers are quarantined for following revelations that a man tested positive for Covid-19 days after completing two weeks of managed isolation.

Epidemiologist Professor Michael Baker says the case, revealed today by the Ministry of Health, is also grounds for considering stricter rules on people's movement in the week after they are released from quarantine to prevent the virus' spread.

The man had arrived in New Zealand from India, and returned two negative test results at the Christchurch managed isolation facility where he had stayed.

He was allowed to fly home to Auckland on September 11, but five days later began developing symptoms of the virus. He subsequently tested positive and infected two close contacts in the community.

"He and his household contacts self-isolated when he developed symptoms. They were all moved into the Auckland quarantine facility on September 18, when the first case returned a positive result," the Ministry of Health said.

Baker said such cases were "very unusual", but had been reported in the past. Genome sequencing would be particularly valuable in figuring out what had happened.

According to the ministry, genome sequencing was consistent with two confirmed cases from the same flight from India to New Zealand that landed on August 27.

Baker said it was documented that some people have unusually long incubation periods, and that it was worth reviewing whether New Zealand should lengthen the isolation period.

"Basically 14 days has always been a practical maximum, but we've always known there was potential for the maximum to be longer than 14 days in very rare cases," he said.

He said it was "extremely unusual" for testing on day 12 of isolation to not have detected the virus.

The person could also have been infected within the managed isolation facility, in which case the genome sequencing could connect him with another case from the facility.

The ministry said it was also possible the man was infected on his flight from Christchurch to Auckland.

"Other passengers from that flight are being contacted and assessed as a precautionary measure to exclude them as the source of infection," the ministry said.

Baker said each option was concerning for different reasons, and questioned whether there should be further rules for people leaving managed isolation.

He was unsure what advice people received upon leaving, but said they could be advised to avoid mingling with anyone other than those in their family group for a week after leaving isolation, and to wear masks.

"Our procedures are going to be right in almost all situations. Now and then we will be tested by events that are right at the limits of what is normal."

He said it was worth reviewing whether changes were made to managed isolation.

The ministry said a 14-day isolation period "remains the gold standard".

"This is also the approach adopted by other countries. Our own modelling confirms that 14 days spent in managed isolation with two tests leaves a very low risk that someone will leave managed isolation with Covid-19.

"This case is another example of the tricky nature of the virus, and a reminder that anyone who has been through a managed isolation facility should remain very aware of their health. Anyone who develops symptoms of Covid-19 should get tested and self-isolate while awaiting results as these people did."