The secret jailhouse witness who gave false evidence in the infamous David Tamihere murder trial is appealing against his sentence for perjury today.

Witness C, who recently lost name suppression and can now be identified as Roberto Conchie Harris, was last year found guilty in a private prosecution by "jailhouse lawyer" Arthur Taylor on eight perjury charges for evidence he gave against Tamihere in the 1990 trial.

But his lawyer, Adam Simperingham, argued in the Court of Appeal in Wellington this morning that the evidence Harris gave was not "critical" to the prosecution's case, and that the sentencing judge picked a starting point of imprisonment that was too high.

Harris, a double murderer, sex offender and fraudster, was sentenced for his courtroom lies to eight years and seven months' imprisonment by Justice Christian Whata.

Witness C, also known as Roberto Conchie Harris, is appealing the length of his sentence. Photo / Peter Meecham
Witness C, also known as Roberto Conchie Harris, is appealing the length of his sentence. Photo / Peter Meecham

He was one of three prison informants during Tamihere's trial for the murders of Swedish tourists Urban Hoglin, 23, and Heidi Paakkonen, 21.

Tamihere, who has always professed his innocence, was convicted and sentenced to life imprisonment, before eventually being granted parole in 2010.

Simperingham said in court today that a starting point of nine years imprisonment for Harris on the perjury convictions was too high, and the judge had wrongly assessed aggravating factors.

"It's for the prosecution to prove aggravating features beyond reasonable doubt," he said.

Harris' convictions on unrelated offending that had nothing to do with perjury should not be considered aggravating either, he said.

The overall sentence was too high, and therefore "excessive".

In his submissions, Simperingham said it was unlikely Harris' lies played any "material role" in the jury finding Tamihere guilty.

Lawyer Richard Francois, who represented Taylor in court this morning, echoed previous courtroom arguments in his opening to the judges, declaring there was "an evil in our legal system".

"For some it is easy to see but difficult to prove. Others don't see it at all," he said.

"This is the first case, as far as we are aware as prosecution, in which a Crown witness has been convicted of perjury in a murder trial to secure a conviction.

"This is a serious matter, so therefore the starting point ... must be steering towards the maximum. Perjury sits alongside the most serious crimes in this country."

Taylor earlier told the Herald he could not imagine a more serious case of perjury.

"The convicted perjurer, who has lied all his life ... [Harris'] sentence is really a merciful one," he said.

He said in court the idea the sentence was manifestly excessive was "ridiculous".

"It could have been 11-14 [years]," he said.

Under the Crimes Act if perjury is committed to procure a conviction the maximum punishment is 14 years' imprisonment.

Harris testified Tamihere, who has other violence convictions, including the 1972 manslaughter of Mary Barchamm, confessed to him while they shared a prison cell.

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

Harris claimed Tamihere attacked and sexually assaulted the Swedes before dumping their bodies at sea.

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.

The Court of Appeal when dismissing Tamihere's appeal of his convictions in 1992 also said: "We would be surprised if the jury had given much credence to any of the detail in the stories Tamihere was said to have told these witnesses."

The judges reserved their decision.