An inmate whose private prosecution led to the perjury convictions of a prison snitch says the police informant claiming his lies didn't matter to David Tamihere's jury "defies common sense".

In a private prosecution, led by "jailhouse lawyer" Arthur Taylor, Roberto Conchie Harris was last year found guilty on eight perjury charges.

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

Previously known only as Witness C, Harris was convicted of perjury for his evidence as one of three prison informants during Tamihere's 1990 trial for the murders of Swedish tourists Urban Hoglin, 23, and Heidi Paakkonen, 21.

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

Roberto Harris was sentenced to eight years and seven months' imprisonment. Photo / Peter Meecham
Roberto Harris was sentenced to eight years and seven months' imprisonment. Photo / Peter Meecham

Last week, Harris' lawyer, Adam Simperingham, filed his submissions to the Court of Appeal in an attempt to overturn Justice Whata's "manifestly excessive" sentence.

In the court documents, obtained by the Herald, Simperingham argues his client's false evidence made no difference to Tamihere's bid for an acquittal.

"Although the jury deliberations leading to David Tamihere's convictions will never be known, it is unlikely that [Harris'] evidence played any material role in finding him guilty," the lawyer said.

But, Taylor told the Herald those claims are "directly contrary to the evidence of David [Tamihere] and [brother and former politician] John Tamihere" at last year's trial in the High Court at Auckland.

"It completely defies common sense," Taylor said.

"Let's understand why [the police] call secret witnesses, it's generally a lack of other evidence so they're considered essential to the prosecution's case."

Lawyer Murray Gibson, who prosecuted Harris on Taylor's behalf, also said at trial that the snitch's false evidence was "powerful" for Tamihere's jury.

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David Tamihere has always professed his innocence. Photo / Jason Oxenham
David Tamihere has always professed his innocence. Photo / Jason Oxenham

Taylor said if Tamihere had been acquitted of the Swedes' murders, "the bottom line is Harris has been found guilty".

"He definitely had a major impact on [Tamihere's] trial and that's what we will be arguing at the Court of Appeal.

"The convicted perjurer, who has lied all his life ... [Harris'] sentence is really a merciful one. I can't imagine a more serious case of perjury.

"A double murder trial, a particularly heinous murder that was on front pages of the country's media for a hell of a long time and had a significant impact on the nation's psyche.

"We'll actually be arguing that he gets a lengthier sentence."

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

Swedish tourists Urban Hoglin and Heidi Paakkonen were murdered in 1989. Photo / File
Swedish tourists Urban Hoglin and Heidi Paakkonen were murdered in 1989. Photo / File

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."

New Zealand's second highest court is due to hear Harris' appeal next month.