Health officials are still racing to work out how a Sydney man caught Covid-19 amid concerns there are chains of transmission that continue to go "unrecognised" in the city.

"(The local case's) contact with the infectious person must have been very fleeting," chief health officer Dr Kerry Chant told reporters on Monday.

"We are concerned there are chains of transmission in the community that are unrecognised.

"We're not out of the risk period."

Chant said the missing link between a returned US traveller and the man from Sydney's eastern suburbs had still not been found as of Monday morning.

The man in his 50s and his wife tested positive to Covid-19 last week.

Neither have links with hotel quarantine, border control or healthcare, sparking concerns about how he caught the disease.

Health officials fear someone may have been unknowingly infectious and going about their business in the community.

NSW Premier Gladys Berejiklian agreed the man's contact with an infected person must have been brief.

She said authorities have combed CCTV and spoken to "lots of people" to try and determine how he caught it, with the man even detailing the roads he had walked along to contact tracers.

"The concern is, if it's one or multiple people, who are those missing links it was a very fleeting meeting," Berejiklian told reporters.

"That suggests there could be a high level of contagion."

New Zealand reopened its travel bubble with New South Wales overnight, with Health Minister Chris Hipkins saying earlier that he was " pleased with the way the response process has been managed".

Hipkins said there was some specific advice for travellers to be aware of. This included:

• Anyone who is in Australia and was at any of the locations of interest at the times specified is asked to follow NSW health advice regarding isolation and testing and must not travel to New Zealand within 14 days from when they were at the location.
• As before, anyone who has arrived in New Zealand and who was at a location of interest at the specified time must self-isolate immediately and call Healthline for information on when they should be tested.

"Our officials remain in close contact with health officials in NSW and will continue to closely monitor the situation. Updates will be provided if there is any change to this latest advice," Hipkins said.

- text by news.com.au. Additional reporting, NZ Herald