Australians unvaccinated against COVID-19 will most likely be prevented from entering New Zealand, even if the trans-Tasman bubble reopens.

The New Zealand government has signalled it is likely to block unvaccinated Australians from travelling under the trans-Tasman bubble - if and when it reopens.

Quarantine-free travel between the Bledisloe Cup rivals is currently on hold, as NZ attempts to keep the damaging Delta variant out.

Last month, Prime Minister Jacinda Ardern popped the bubble for eight weeks until September 18, with a decision on reopening to come early next month.

Given case numbers in NSW and Kiwi fears around Delta, COVID-19 Minister Chris Hipkins said a bubble remodelling beckoned.

"It is unlikely that the trans-Tasman safe travel zone will simply reopen in exactly the same form as it was before all of this happened," he said.

"Vaccines by that point will be more widely available on both sides of the Tasman.

"It may be that vaccinations become a prerequisite for non (NZ) citizen travel."

Mr Hipkins said legal challenges were likely to prevent the no vax, no fly policy applying to Kiwis.

The bubble change is foreshadowed in declassified documents commissioned by Ms Ardern's government on NZ's reopening to the world.

On Wednesday, the government released three pieces of cabinet advice prepared by the expert group driving their COVID-19 response.

The papers strongly suggest "Fortress New Zealand", with widespread border closures, should continue through to next year.

They also paint a grim picture for any Kiwis who believe normality could be around the corner.

NZ hopes to vaccinate every consenting adult Kiwi this year, but the advice suggests the rollout could be less effective should a more damaging variant emerge.

"It is not inconceivable that, by the end of the year, there could be an established variant that is significantly resistant to the vaccine," the advice states.

"No-one knows what the outcome of this pandemic will be in say three-to-five years' time."

The papers - authored by University of Otago epidemiologist David Skegg and other authoritative figures - does not suggest setting a target on vaccination rates before opening borders, arguing authorities should aim to "as close to 100 per cent as possible".

Advice provided in June outlines less restrictive quarantine arrangements - known in NZ as "MIQ" - which could include home isolation with enhanced testing, proposing to trial it amongst fully-vaccinated Kiwi travellers.

Those plans are thrown out the window in advice given in July - showing the evolving nature of the virus and the policy response.

"In the seven weeks since ... the global situation has deteriorated significantly," Dr Skegg writes.

"It is sobering to see what apparently resulted from the infection of a single person with (the Delta) variant in Sydney.

"Even with current settings, New Zealand is liable to experience an outbreak similar to that in NSW over the coming months.

"We could not recommend rolling out this scheme until as many New Zealanders as possible have been vaccinated."