New Zealand has paused flights from Australia's biggest city after learning of two community cases of COVID-19.

New Zealand has quarantined a traveller who visited Sydney exposure sites from a recent COVID-19 community case.

A person from Christchurch is being tested for the virus and transferred to a managed isolation facility after NSW health authorities identified them as a recent case.

The person is not symptomatic, and is being isolated as a precaution.

New Zealand has suspended travel from NSW for 48 hours as a result of the new Sydney cases.

Kiwi health authorities are keen to know more about how the virus made its way from a positive case at the NSW border to the new community case - the "missing link" - before restarting travel.

The suspension will be reviewed on Saturday, and is currently due to be lifted at midnight on Saturday.

In the meantime, NSW contact tracers are trying to identify the missing link, and Kiwi contact tracers are getting in touch with recent travellers from NSW.

The health ministry said on Friday it had contacted 5214 people who had recently flown from Sydney, giving them advice to monitor their symptoms.

The NZ government has also clarified the travel suspension applies to anyone "who has been in NSW (as of Thursday night) and who plans to depart for New Zealand from another airport eg Brisbane or Melbourne".

This restriction therefore bans residents far from Sydney's exposure sites - such as Tweed Heads, over 800km away from the capital - from travelling to NZ.

The nation's top doctor said the risk to Kiwis from the two new cases was "small" but enough to justify the suspension of quarantine-free flights.

Director-General of Health Ashley Bloomfield said the "line-ball call" was made as there were "a few unknowns", specifically relating to how the pair caught the deadly virus.

"It's good that they have been able to link these two new cases, the husband and wife, to a border case and someone who is in managed isolation," he told Radio NZ.

"It's good that it's recent. It's not like there's infections bouncing around in the community.

"But there is still a missing chain of transmission there, a missing link as it were."

People can still travel from Aotearoa to NSW without quarantining - though airlines have cancelled flights, making that practically difficult.