For the past 19 years, despite two trials and a Privy Council ruling, debate has continued to rage about whether David Bain shot his family, or whether his father Robin committed the murders before shooting himself.
On TV3's 3rd Degree programme, reporter Melanie Reid showed that Robin Bain's finger and thumb, photographed at the crime scene, had marks which looked remarkably like the gunpowder deposits left after loading a .22 rifle. The photo was one of 2500 taken at the scene.
David Bain's lawyer, Michael Reed QC, says the evidence the evidence proves his client is innocent, and that the government should pay compensation.
Ms Reid herself loaded and tested .22 rifles with the same sort of operating systems as the murder weapon and using the same ammunition. She found marks on her thumb, two grey parallel lines very similar to those seen in the police photos of Robin Bain.
3rd degree also showed footage of David Bain's defence team testing, under police supervision, the actual murder weapon, a Winchester 490 .22 semi-automatic. The members of the team who fired the gun also found lines of gunpowder residue on their thumbs.
On the TV3 show, two gun experts testified that the marks on Robin Bain's were instantly recognisable as having come from loading a rifle.
One of them, gun-shop owner and marksman Richard Munt, said gunpowder deposits only remain on the skin for a few minutes, so they could not have been caused by Robin Bain using the rifle the previous day.
"He would have to have been shooting a rabbit a minute or two before he was found, and that was unlikely."
Forensic photographer Peter Durrant and gunsmith Robbie Tiffin both measured the Robin Bain marks and found they matched exactly the lips of the murder weapon's magazine.
Ms Reid explained that 3rd Degree had also shown the photographs to an international weapons specialist with almost 20 years' experience as leader of a tactical firearms unit of the UK police.
He has testified: "In my expert opinion the marks seen on the finger and thumb, in the photograph, are consistent with loading and unloading a .22 magazine which has carbon deposits upon it."
The marks were first spotted by David Giles, an eagle-eyed Waikato businessman with no previous connection to the Bain case.
Mr Giles said: "Anyone who has handled guns would probably recognise marks like that."
Ms Reid told 3rd Degree presenters Duncan Garner and Guyon Espiner that in her view the evidence was a game-changer.
Watch the episode here.
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