New Zealand should not extradite a murder suspect to China because assurances he will not be tortured or face the death penalty can not be trusted, the Green Party says.

A legal challenge has been mounted to overturn Justice Minister Amy Adams decision to send Korean-born New Zealand resident Kyung Yup Kim to Shanghai, and the Greens human rights spokeswoman Marama Davidson says Mr Kim should stay in New Zealand.

"There is no way the Minister can actually guarantee Mr Kim will receive a fair trial, won't be tortured and won't be subject to the death penalty," Ms Davidson said.

"New Zealand has signed up to the United Nations Convention against Torture and as such we are not supposed to send people to countries where they could be tortured by the state."

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Just last week, the United Nations said that 'the practice of torture and ill-treatment is still deeply entrenched in the criminal justice system' in China.

"Human rights barrister Tony Ellis has filed for a judicial review of the extradition decision.

Dr Ellis has also filed an application for bail, to be heard in the High Court at Wellington today, and an application to discharge the extradition because of the length of time Ms Adams took to make her decision.

He told the Herald that China's human rights record meant there was a real possibility Mr Kim would be tortured upon his arrival. Further, a guarantee of a fair trial was impossible.

"The validity of the assurances is highly questionable, as is whether you can enforce the agreement or monitor it properly."

Mr Kim, 40, has been held at the Mt Eden Corrections Facility since an Auckland District Court judge issued a provisional warrant for his arrest in June 2011.

The warrant was issued at the request of Chinese authorities, which are seeking Mr Kim's extradition over the murder of 20-year-old Peiyun Chen.

Chinese criminal investigators allege Mr Kim killed Ms Chen at his Shanghai home on December 11, 2009, before dumping her body in a wasteland and fleeing to South Korea.

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Mr Kim returned to New Zealand, where he has held permanent residency since age 14, in October 2010.

China asked that he be sent back but because there is no extradition treaty between New Zealand and Beijing the request went to then-Justice Minister Simon Power.

Mr Power allowed the case to go to court, starting a drawn-out legal process that eventually returned the decision to Ms Adams.

Ms Adams said: "I have sought, and received, undertakings from the Chinese Government waiving the death penalty should Mr Kim be convicted and providing for his monitoring, fair treatment and trial.

"I have also given careful consideration to Mr Kim's submissions [provided during] the process."After considering all the relevant material and in light of the PRC's assurances, I was satisfied none of the mandatory or discretionary grounds in the act prevented his surrender."

Dr Ellis said: "Mr Kim will fight to the last-ditch. We probably have a year or more to go with the remedies, but I think we have a reasonably strong case that the minister's decision is wrong."

A judicial review hearing would likely be in February next year. If that went against Mr Kim, he would appeal to the Supreme Court if necessary.

"If he loses that, I think we will probably be the first case that would apply for interim measures before either the UN Human Rights Committee or the torture committee."

Dr Ellis said part of the judicial review application would be the "extraordinary" amount of time that Mr Kim had been held in prison, which had caused him to become depressed and suicidal.