Labour justice spokeswoman Jacinda Ardern disputes claims by John Key and Amy Adams that an independent criminal review commission would replicate what New Zealand has now and that the country's justice system is robust enough.

Both the Prime Minister and the Justice Minister said that 99.9 per cent of convictions were upheld on appeal.

They were speaking in the wake of another high-profile criminal conviction being overturned by the Privy Council: Teina Pora's conviction for the 1992 murder and rape of Susan Burdett.

Since the Supreme Court was formed in 2004, it has been the final appellate court for New Zealand convictions after that date and once appeals are exhausted, an application can be made to the Governor-General for a Royal Prerogative of Mercy.

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Pardons granted under that are essentially made on the advice of Ministry of Justice officials to the Minister of Justice and are very rare.

Ms Ardern said that an independent commission would not replicate what happened now.

The Royal Prerogative of Mercy to the Governor-General essentially meant making an application to the Ministry of Justice and the Minister of Justice.

"It's hardly an independent process if you are actually making an application to a political individual to make a decision around a miscarriage of justice."

The British Criminal Cases Review Commission, which has been running since 1997, could send a case back to an appeal court, and its finding would be used as the basis of a quashing or a retrial.

"It does speed up the process but it also adds an independent element."

Mr Key said yesterday there was already a proper process.

Ms Adams said the few convictions that were overturned demonstrated the system worked as it should with the review of decisions of lower courts being under the jurisdiction of higher courts in the appellate system.

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"From time to time the issue of setting up a Criminal Cases Review Commission has been raised with successive Governments but none have progressed the idea."

She said establishing an independent body would essentially just replicate the process already available through the Royal Prerogative.