Michael Woodhouse said he was acting within international law.

A former Zimbabwe secret police officer who has admitted to murder must leave on his own accord, the Immigration Minister says.

William Nduku, his tribal name, arrived in New Zealand in 2015.

He was refused asylum or the right to work and study and has been forced to survive on handouts from friends and the expat community and could face death if deported back to Zimbabwe.

RUETERS The man operated as a secret police officer and murdered for Robert Mugabe.

On arrival to New Zealand, the 31-year-old said he immediately informed Immigration New Zealand that he'd entered the country under an assumed identity and was seeking asylum.

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He said at 19, he was forced to serve in Zimbabwe President Robert Mugabe's secret police and participated in up to 20 murders, several rapes and multiple tortures.

PHILIMON BULAWAYO/REUTERS Robert Mugabe's regime has been known to crush dissent through any means necessary. Nduku was forced to work for the president's secret police when he was 19.

On Wednesday Immigration Minister Michael Woodhouse said New Zealand was a signatory to the 1951 refugee convention that prevented asylum being granted to people who had committed war crimes or crimes against humanity.

But it was also impossible under international law to repatriate them if there was a legitimate fear for their lives.

"The only question I have to answer is whether or not I legitimise their stay in the country by the grant of visa or residence.

"Decisions have been made in a certain case and I stand by those decisions."

He was satisfied that the circumstances that led to Nduku's arrival would not be repeated and that Immigration New Zealand had made fair and careful decisions in the case.

Woodhouse accepted New Zealanders would have concerns about Nduku being in the country.

"They'll have a legitimate concern and I'm quite sure officials will be monitoring situations of that nature, but as I say I have to comply with international law and that's what I've done."

Nduku had the option to leave on his own accord, Woodhouse said.

Nduku spent several months in Mt Eden prison. While in prison he alleged members of the Mongrel Mob assaulted him on a regular basis.

"It was awful. I wouldn't wish that experience on anybody."

Nduku said he had been living off the generosity of others in Auckland, forced to try and make a living without the right to work.