Trans-Tasman bubble: Thousands of New Zealanders being chased by health authorities after NSW COVID-19 case closes border

Thousands of New Zealanders are being monitored and contacted by public health services after a mystery case of COVID-19 closed the travel bubble to New South Wales.

Dr Ashley Bloomfield told The AM Show on Friday it's unlikely any returning New Zealanders will have been in contact with the positive cases - a husband and wife in Sydney - but caution is being exercised regardless.

Between 5000 and 6000 people have returned from New South Wales since the cases were identified and Bloomfield says they are all being chased up.

"The likelihood anyone was in the places of interest at the times specified is rare but we're contacting them all to make sure they know those locations."

The two Australian cases are genomically linked to a man still quarantined in a border facility who contracted the Indian variant of the virus. However, there is a missing link, as it's unclear exactly how the husband and wife caught the virus.

"We're still not quite sure of the link between the two new cases. We think the man got infected first [and then his wife] but we're not sure of the link between him and this case in managed isolation who returned from the US," says Bloomfield.

The travel bubble will be paused until authorities know more about this missing link, he continued.

"We just want a few more days of testing… what's served us well to date is just taking a precautionary approach."

The decision to pause travel between New South Wales and New Zealand is being praised by Professor of Immunology at Murdoch University Cassie Berry.

She says stopping travel is "a very responsible decision".

"It's quite likely there possibly has been other people infected and they may have spread it to others," Berry told Newshub.

"Not everyone infected with this virus is equally contagious. There are some people who can shed the virus, we call them super-spreaders, and that is difficult to contain if they're unaware they are infected."