A leading epidemiologist says he's "quite surprised" with the Government's decision to pause all quarantine-free travel with Australia. 

COVID-19 Response Minister Chris Hipkins announced on Saturday night the trans-Tasman travel bubble would be suspended for three days.

It comes as the coronavirus outbreak in Sydney worsens, with cases also being detected elsewhere in the country.

The pause came into effect at 10:30pm on Saturday night and will last until 11:59pm on Tuesday.

But epidemiologist Michael Baker says given the fact "the green zone with Australia, in general, has worked well", he's "quite surprised" by the move.

He said although there was a "strong argument" to limit travel from states on the eastern side of Australia where risk is higher, there are still parts of the country where the virus remains under control.

"I'm more surprised that it's a general restriction on all of Australia because some states and territories there have been as good as New Zealand at maintaining elimination - I'm thinking about, particularly, Western Australia, Tasmania, South Australia, Northern Territories," Prof Baker told Newshub.

The travel pause comes after a tourist from Australia tested positive for COVID-19 after returning from a trip to Wellington last weekend. It was later confirmed he had the Delta variant of the virus.

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Wellington is currently at COVID-19 alert level 2, and so far there have been no community cases recorded in New Zealand following the scare.

Cabinet is set to meet on Sunday to decide whether to lift or extend the restrictions in place in Wellington.

Prof Baker said due to the greater risk posed by the spread of the highly infectious Delta variant, there would be "very serious consequences" if a new outbreak did occur here as a result of the trans-Tasman bubble - so the travel pause was "understandable on that basis".

He said the Government's decision to suspect all quarantine-free travel might also have been influenced by other factors relating to how well Australia is able to control the spread of the virus within its own borders.

"It may reflect a lack of confidence in the quality of [the] surveillance in some parts of Australia or the extent to which states there have been able to limit transmission between states.

"I think we will obviously wait and see to see what the full rationale for this is, but this is quite a strong measure to take at this point given that the trans-Tasman green zone has been working well in general apart from just recently."

In announcing the travel bubble pause on Saturday night, Hipkins said it would allow the Government "to get a better understanding of developing a situation and to consider the potential implantation of a range of measures to make the bubble safer".

This could include pre-departure testing for all flights from Australia to New Zealand, he said.