Data has emerged showing just three people who travelled directly from Australia to New Zealand tested positive for Covid-19 over eight months.

It has been greeted by experts as a positive step towards a trans-Tasman bubble - but they say New Zealand needs to lift its game at the border to Australian levels to reduce the risk of creating a Covid-19 outbreak.

The data showing the three positive cases came through an Official Information Act request to the Ministry of Health seeking the number of positive Covid-19 cases since March last year. It showed that there were 39 positive cases in March and April last year - then almost nothing.

A case was identified in August, one in November and another in January. Statistics NZ data shows that over the same period 23,447 people travelled from Australia to New Zealand.

Otago University epidemiologists professors Dr Nick Wilson and Dr Michael Baker were supportive of the data adding strength to arguments for a bubble but added a note of caution. The academics last year carried out research into alternative ways of managing incoming travellers.

Wilson said "thoughtful" systems were needed for quarantine-free travel between Australia and New Zealand.

Those included developing processes so each incoming Australian traveller was required to download the Ministry of Health app for scanning QR codes, activating its bluetooth function and agreeing to use it on every occasion during the first few weeks.

He said saliva tests at the airport would also reduce risk, as would a ban on attending large-scale events and carrying out spot checks on travellers with fines for those who ducked the rules.

"I would hope they would be discussing these things now. The economic benefits are enormous.

"In general, I would be more concerned with New Zealand's border control than Australia's. We just have such a loose process."

He said quarantine rules in Australia offered a higher level of protection from the virus, including rules such as arrivals being confined to their rooms after arriving. In New Zealand, risk areas included bus trips for exercise, movement within MIQ facilities, smokers mixing in outdoor areas without masks, and other areas.

He said New Zealand's approach had become locked in "path dependency", meaning the government struggled to deviate from the system developed as the pandemic approached.

Wilson said the vaccination programme in both countries was also encouraging and as the percentage of the vaccinated population grew, so did confidence in freeing up travel.

Baker said the figures were "very supportive evidence" for a trans-Tasman bubble. It didn't suggest risk-free travel but new systems - such as those outlined by Wilson - could reduce the risk to a manageable level.

"It's never 'no risk'. There will always be a risk. It's about managing it successfully."

He said he preferred to use the term "green zone" to identify lower-risk countries, while those with high risk were in the "red zone". He said greater work in "red zone" countries to manage those travelling to New Zealand would reduce the chance of border breaches.

"This is a chance to benchmark our border against Australia's. The world needs models of success. It's a great opportunity to show a model that could be rolled out globally."

Baker said those travelling would have to accept that a community outbreak where they were visiting could lead to becoming trapped there.

"That will be the trade-off - all travellers will have to go into this with their eyes open. Things can change very quickly."

University of Auckland associate professor microbiologist Dr Siouxsie Wiles said the data showing three infected travellers from Australia was heartening.

"There is always the complication that not every person infects someone else - and some go on to be super-spreaders."

Wiles said it was important those travelling from Australia were not sharing a plane with those in transit from other countries because of the chance of contracting Covid-19 while flying.

She said one possible risk to be managed would be the opening up of spaces in MIQ and the increase in travellers from less safe countries.

Health minister Chris Hipkins said he was aware of the figures and Australia was seen as lower-risk for cases of origin, which was why so much work was going into a trans-Tasman bubble.

"But as we've said previously, low risk isn't no risk. While we've always been clear we'd want to open up this particular bubble as soon as we were able, that wasn't going to be before both countries felt equally comfortable around risk."

Hipkins said the time taken working on travel to Australia had strengthened New Zealand's position for a "broader reopening of travel", including issues such as insurance, clear messages for those having to seek shelter, and the impact on airlines and airports facing higher numbers of arrivals in a Covid-19 environment.

"Solidifying our position in regards to travel between here and Australia will help us set an ongoing template for the future."

BusinessNZ chief executive Kirk Hope said there was eagerness for a travel bubble with Australia.

"But if the cost of that is increasing alert levels in Auckland, it's a zero-sum game. The economic cost outweighs the benefit."

Act Party leader David Seymour said questions remained over the quality of New Zealand border controls even as Australia had improved its systems.

Seymour said New Zealand needed to stop viewing the world as having equivalent risk across the globe and developing a system responsive to Covid-19 "hot spots".

Until it did, he said it echoed Australia's question: "New Zealand, where the bloody hell are you?"

A Ministry of Health spokesman said the data - along with other information - was used to develop New Zealand's response to Covid-19. "This includes the potential for any travel bubble and when this may be able to occur."

The data showing the three positive cases was compiled through the ESR database based on people being asked for the last three countries in which they had spent time.