The Prime Minister isn't ruling out sending the Christchurch mosques terrorist back to Australia to serve his jail sentence, despite concerns from the Opposition it could justify that country's deportations to New Zealand.

Jacinda Ardern says cost won't be a determining factor in keeping the gunman in New Zealand, rather it will be up to the desire of the victims.

It was revealed on Tuesday documents recommended Cabinet approve $3.6 million in operating funding to keep Brenton Tarrant behind bars. So far, $1.9m has been approved to keep him in prison for this fiscal year, with a further $1.6m for next year.

Ardern says it's a "high-cost case" due to the extra precautions that have been put in place.

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

• Christchurch mosque attacks: Accused pleads guilty to murder, attempted murder and terrorism
• March 15 terror attack remembrance event cancelled
• Outrage over initials on bullet-shaped pendant
• ‘She’ll be right’ factor is our biggest threat

"Usually it's the case that someone will see their sentence in the country in which they're sentenced. So that's one of the major features there."

But National leader Judith Collins says if Tarrant is sent to Australia to serve his sentence, it could justify the country sending detainees with Kiwi citizenship - known as 501s - back to New Zealand after they've completed their sentence.

"Last time I checked - [there were] around about 900 New Zealanders in Australian jails. Would you like us to have all those back?"

The controversial deportations have occurred since 2014, and the Australian government has sent thousands of people with Kiwi passports to New Zealand even if they have little or no connections here.

"I also believe he committed his evil crime here. I think he needs to have his sentence here, even though it costs us a lot of money… I think we take ownership of the fact that he committed his crime here and he will serve his sentence here."

Tarrant is currently awaiting sentencing in Paremoremo Prison for killing 51 people when he opened fire at two Christchurch mosques in March last year.

His sentencing hearing is due to begin later in August.