New Zealand is set to announce on Monday whether it will open quarantine-free travel to Australians, but Prime Minister Jacinda Ardern indicated that such an arrangement may only be with some Australian states.

Ardern told state broadcaster TVNZ in an interview that initial country-to-country negotiations had turned to state-by-state discussions as the process was taking too long.

"We’ve said: ‘Look, let's just move state-by-state’ because it’s actually just taking a bit too much work, a bit too difficult,” Ardern said.

“Let’s just operate as Australia has been operating with us. That’s helping to speed things up.”

Australia's border has been mostly open to neighbouring New Zealanders since last October, with a few short suspensions when there were small coronavirus outbreaks in Auckland. But New Zealand has delayed returning the favour amid more frequent bursts of COVID-19 clusters across Australia.

Ardern did not give any details on when such a travel arrangement was expected, but local media have reported it may be operational by the end of April. The prime minister is expected to announce her decision in a post-Cabinet news conference later on Monday.

Pressure has been mounting on Ardern's government to allow Australians entry as the country's tourism sector struggles without international visitors. The opposition National Party is calling for quarantine-free travel with Australia to start immediately.

Australian Prime Minister Scott Morrison said earlier this month that a two-way travel arrangement was in New Zealand's hands