David Tamihere's double-murder case is heading back to the Court of Appeal, Justice Minister Andrew Little says.

This follows acceptance by the Governor General of advice from Little last year on a Royal Prerogative of Mercy application made by Tamihere.

Tamihere spent nearly 20 years in prison after being found guilty in 1990 of murdering Swedish backpackers Sven Urban Hoglin, 23, and Heidi Paakkonen in the Coromandel.

He has always professed his innocence.

Tamihere had skipped bail for a 1986 rape and was on the run when the murders were committed. The brother of former Labour Cabinet Minister John Tamihere also has a manslaughter conviction for the 1972 death of an Auckland prostitute.

Little made the announcement today after considering Justice Ministry advice, assisted by a retired High Court judge.

"I advised Her Excellency to refer Mr Tamihere's convictions back to the Court of Appeal for further consideration," Little said.

"As the matter will shortly come before the Courts, I will not be making any further comment," says Little.

The case will be heard by the court as a further appeal, Little said.

Little's announcement comes after a police informant at Tamihere's 1990 trial was accused and found guilty of perjury after a 2017 trial in Auckland.

Former inmate and "jailhouse lawyer" Arthur Taylor's private prosecution of the prison snitch, who was only known for decades as Witness C, uncovered a string of lies and suspect police work.

Witness C was one of three prison snitches that the Crown relied on at Tamihere's trial to give what was described as "powerful" evidence.

After a lengthy suppression battle, Witness C was finally unmasked in April 2018 by the Herald and other media as Roberto Conchie Harris.

Harris, a double murderer, sex offender and fraudster, was sentenced to eight years and seven months' imprisonment for the eight perjury convictions.

Tamihere has told the Herald that Witness C's perjury convictions were a "major" moment in his case.

At the 1990 trial, Harris testified that Tamihere confessed to him while they shared a prison cell.

Harris was serving time for killing Northland couple Carole Anne Pye and Trevor Martin Crossley when Tamihere was arrested for the Swedes' murders.

He told the jury Tamihere attacked and sexually assaulted the Swedes before dumping their bodies at sea.

But in 1991, Hoglin's remains were discovered in bush near Whangamata, about 70km from where the murders were said to have taken place.

Paakkonen's remains have never been found.