New Zealand could now open up quarantine-free travel with several Australian states, a leading epidemiologist in Melbourne says.

"It is safe for New Zealand to have quarantine-free arrivals from most of Australia," Professor Tony Blakely, a public health medicine specialist at Melbourne University, told the Herald.

"In fact, it has moved beyond that and New Zealand is now dragging the chain, compared to many states in Australia allowing quarantine-free arrival of Kiwis."

But Prime Minister Jacinda Ardern made no apologies for having a lower tolerance for cases than Australia, nor was she satisfied about the threshold for shutting down inter-regional travel in Australia.

There were only eight cases in Australia in the past 24 hours, most of them imported quarantine cases, while "Covid hotspot" status is set to be lifted tomorrow in Victoria and New South Wales.

Blakely said Victoria and New South Wales had already met the elimination criteria of no community cases from an unknown source for 28 days, while Queensland, Tasmania, Western Australia, Australian Capital Territory and the Northern Territory had long achieved it.

As of tomorrow, all states and territories were allowing quarantine-free travel other than South Australia (WA has risk-based home quarantine rules).

"South Australia will – I strongly suspect – regain elimination status soon," Blakely said.

"States that have delayed [quarantine-free travel] have been playing politics, not science. New Zealand is now in that camp, in my view, and has no reason to not join a travel bubble with all states and territories in Australia other than SA, for now."

The bubble would not only provide an injection for tourism and the economy in general, but it would also free up about 40 per cent of the beds in New Zealand managed isolation and quarantine facilities.

Blakely said any new mystery cases in the community would see a state or country drop out of the transtasman bubble until Covid is stamped out again.

Professor Tony Blakely, a public health medicine specialist at Melbourne University, says New Zealand is dragging the chain on the transtasman bubble. Photo / Supplied

But Ardern said there needed to be stricter rules around restricting travel from a "Covid hotspot" because of the risk of a Covid-carrier travelling to a Covid-free state and then flying to New Zealand.

The hotspot criteria was previously 30 cases in three days, but Ardern said that was too many.

"You might get into a place where we're free of community transmission, but actually it's as important for us to recognise what will happen if and when cases arise, because they will and they do."

She agreed that the ball was in New Zealand's court, as Kiwis could already travel to NSW, ACT, NT and Victoria without having to quarantine.

"They've already opened up, so from their perspective, it's already done and dusted. We're the easy partner in this. We have a lower tolerance for cases."

A bubble should have domestic borders acting as a buffer, and operate in a way that didn't leave travellers stranded, she added.

"We don't want to yo-yo in and out of travel with Australian states."

Asked if a bubble might still be out of reach if Australia was Covid-free but those hotspot rules were still too loose, she said: "That's a hypothetical we're not in yet."

She added that traces of the virus were found at a Melbourne wastewater facility 10 days ago.

Blakely also suggested allowing fewer days in quarantine for overseas arrivals from countries with low levels of infection.

A similar approach has also been suggested by public health experts in New Zealand - dubbed a risk-based "traffic-light" border system - which includes a pre-departure negative test and quarantine period for travellers from high-risk countries.

But Ardern has repeatedly said Kiwis need to be able to exercise their right to come home.