A former police officer says a key witness, whose testimony helped convict a woman of murder, told him at the time they had lied to detectives.

Now Gail Maney hopes the new information will help put an end to what she believes is a 20-year injustice.

Andrew Thompson, a former Henderson Police officer, told the Herald on Sunday he collected the witness from an interview with detectives investigating the death of Deane Fuller-Sandys and drove that person to the airport.

"As soon as [suppressed] got into the police car [they] looked at me and said: 'Nothing I told those detectives is true'.

"We had a discussion about the importance of telling the truth. I thought I had absolutely convinced [them] to tell the truth."

It was 1998 and the then uniformed officer said he was unsure if the witness was defending or incriminating Maney. His main concern was that they not lie.

Tim McKinnel addresses media after Malcolm Rewa was found guilty of murdering Susan Burdett earlier this year. Photo / Jason Oxenham

Maney, 51, was twice convicted of putting a "hit" on Fuller-Sandys who failed to return from a fishing trip at Whatipu in 1989 and was originally believed drowned for almost a decade.

Police said Maney, a former prostitute, ordered him killed after he supposedly burgled Maney's flat on Larnoch Rd in Henderson.

But she has always maintained her innocence and claims she never met him.

Now her case is being investigated by former police officer turned private investigator Tim McKinnel, whose years of research led to Teina Pora being cleared of a murder conviction.

McKinnel told the Herald on Sunday he believed Thompson's account which was consistent with other information he has.

"I think it's an important piece of new evidence because it goes some way to corroborating what a now retracting witness is saying.

"In this case it not only helps provide context, but it demonstrates that the witness hasn't made this up in recent years."

The witness, who has permanent name suppression, has signed an affidavit saying they lied to police by admitting to being present during Fuller-Sandys murder at Maney's home.

Gang member Stephen Stone was convicted of carrying out Fuller-Sandys murder and of raping and murdering prostitute Leah Stephens five days later, because she had witnessed the shooting.

Stephen Stone before he was convicted of the murders of Dean Fuller-Sandys and Leah Stephens. He remains in prison while Gail Maney is fighting to clear her name. Photo / Martin Sykes

Stone, Maney, her younger brother Colin and Mark Henriksen were all convicted in 1999 for their parts in the murder.

Maney appealed but was convicted again at retrial in 2000 and further pleas to the Court of Appeal in 2003 and Supreme Court in 2007 were dismissed.

"The Crown theory and the convictions rest on four witnesses who said this murder happened," McKinnel said. "We now have two of those who have retracted."

The other retracting witness is Tania Wilson, Maney's old flatmate, who admitted in 2005 she had lied under oath.

Both Wilson and the second witness claimed they were harassed by police and threatened that their children would be taken off them if they did not co-operate.

Deane Fuller-Sandys was a keen fisherman who disappeared in 1989 after going to Whatipu Beach on Auckland's west coast, to fish on August 21, 1989. Photo / File

The other witnesses were two men who told police they helped Stone rape and murder Stephens, who disappeared from Queen St in 1989. Her remains were found at Muriwai.

"They were granted immunity from prosecution for raping and murdering Leah Stephens, which is extraordinary," McKinnel said.

In a further twist, the police officer in charge of the investigation into Fuller-Sandys death, Mark Franklin, was jailed for nine months in 2013 for selling drugs in Rarotonga.

McKinnel said yesterday Maney was pleased with the revelation, but also conscious of how it might affect others involved in the case including the family of Fuller-Sandys, who was 21 when he disappeared.

His body was never found and in both murders there was no crime scene, no DNA and no ballistics, leaving police to rely solely on witness testimony.

"She takes a lot of heart from the fact people are prepared to stand up all these years later and have the courage to say what happened."

Maney, who was released in 2010 but recalled twice and released in mid-2017, was instructed by her lawyer not to speak to media.

The Larnoch Rd house in Henderson in 1999, where police said Deane Fuller-Sandys and Leah Stephens were murdered a decade earlier. Photo / Martin Sykes

McKinnel, whose investigation was instrumental in quashing Teina Pora's convictions for the rape and murder of Susan Burdett, says Maney's case is "more extraordinary".

"Aside from believing Gail, I've spent close to a year looking at all of the evidence, the circumstances of the case, the red flags that exist, and I've seen no evidence that convinces me that Gail was involved in Deane Fuller-Sandys' murder and no convincing evidence that any such shooting took place.

"The Crown case if you accept the retractions, rests entirely on the evidence of two secret witnesses who were granted immunity for the prosecution, who have told multiple versions of events and repeatedly lied to police."

Police were aware the retracting witness had come forward over their evidence, and their allegation they provided information under duress. 

"These matters will be the subject of assessment as will any other new information relevant to the death of Deane Fuller-Sandys in 1989," a spokeswoman said.

She would not comment on what steps they might take over the matter.