However, it's unclear whether a legal framework or political appetite exists for the transfer of New Zealand's worst mass shooter - and now the country's most expensive inmate.

Deputy Prime Minister Winston Peters has called on Australia to take custody of Brenton Tarrant, raised in the NSW town of Grafton, saying "he should be in Australia".

"I'm putting my cards on the table saying 'Go on, take him back into your prison environment'," Mr Peters told AAP.

"That was my view from day one but of course, it was sub judice so I couldn't say anything while he was waiting for trial.

"After he's been sentenced that's what I think should happen."

Prime Minister Jacinda Ardern isn't as steadfast, saying she would "never have taken that off the table" but "we do need to make sure that justice is done here first".

For Tarrant to serve his term in Australia, both governments either side of the Tasman would need to agree, and NZ would need to accede to a transfer convention.

Freshly released Cabinet papers in Wellington have revealed the high cost of housing Tarrant, who shot and killed 51 people on March 15 last year.

A total of $NZ3.59 million ($A3.33 million) has been approved for the first two years of Tarrant's stay at Auckland's Paremoremo Prison.

Additionally, $NZ790,000 ($A733,000) has been set aside this year for screening incoming and outgoing mail for high-risk inmates.

That measure has been directly attributed to an unfortunate incident last year when one of Tarrant's letters - reportedly sent to a contact in Russia - was posted on a notorious online forum.

Ms Ardern said the bill to house the 29-year-old was unfortunately necessary.

"It is a high-cost case," she said.

"Obviously we've already seen what happens if we don't monitor closely, for instance, the correspondence that that individual is is engaging in, and of course some of the other precautions we need to put around the terrorist."

Australia and New Zealand are close allies but are bitterly split on the issue of deportations.

The Australian government has deported thousands of people with Kiwi passports to NZ since 2014, including many who have little connection to Aotearoa.

Ms Ardern has been resolutely critical of the deportations, telling Prime Minister Scott Morrison in Sydney in February, "Do not deport your people and your problems".

On Tuesday, she drew a distinction between a prisoner relocation and the thorny issue, saying Australia's deportations occurred at the end of sentences.

Tarrant will be sentenced at Christchurch's High Court in three weeks, in the middle of a national election.

Opposition Leader Judith Collins, who was corrections minister under former PM John Key, defended the multi-million dollar price tag to house Tarrant.

"It's a real temptation for people to say 'Send him back to Australia before he's served a sentence'," she said.

"I'd say to (those people), there are around about ... 900 New Zealanders in Australian jails. Would you like us to have all those back?

"He committed his evil crime here. I think he needs to have his sentence here, even though it costs us a lot of money."

Australian Associated Press