A TV documentary which claimed to have new evidence that Robin Bain could not have killed his family was last night labelled "unadulterated rubbish" by David Bain's legal team.

They confirmed that they had tried to get the documentary postponed, while David Bain fights for compensation.

But supporters of Robin Bain believe the documentary will change public opinion and help in their fight to prevent David Bain receiving any compensation for the 13 years he spent in jail.

The documentary, by Bryan Bruce, called into question some of the testimony given by defence witnesses at the 1995 murder trial and 2009 retrial, in which Robin Bain was portrayed as a depressed man with alcohol and sexual problems.

David Bain's legal team have always argued that Robin shot dead his wife Margaret, son Stephen and daughters Laniet and Arawa in Dunedin in 1994, before turning the rifle on himself.

They claimed Robin was having an incestuous relationship with Laniet, and feared this being exposed.

David Bain, 38, was last year acquitted of the murders.

Mr Bruce last night singled out the evidence of Daryl Young, who had told David Bain's retrial last year that he sold Robin Bain a school photocopier in 1993.

As part of the defence team's claims that Robin Bain was in an incestuous relationship, Mr Young gave evidence of coming across Robin Bain's van in a camping ground, and hearing voices inside which he believed to be Robin and a female.

Robin Bain came out wearing only a towel and smelling of stale alcohol, Mr Young told the court.

But Mr Bruce last night produced the sales document for the photocopier. It had the signature of Robin Bain and a different salesman, Ian Arthur.

Mr Arthur told the TV show that he had made initial contact with Robin Bain and had made the sale in 1993.

He said Mr Young, a colleague, may have accompanied him on one trip to see Robin Bain but he did not recall any occasion in which the school principal was seen half naked.

Mr Young told the documentary: "I have moved on, mate." He said the sale may have gone through in Mr Arthur's name, "but I made it".

Mr Bruce also said there was no forensic evidence linking Robin Bain to any of the murdered family and his documentary showed the questionable chain of actions Robin would have had to have performed - including shooting himself without leaving any of his bloody finger prints on the gun. "David got a fair trial last year. His father did not."

Mr Bruce said witness discrepancies needed further investigation.

"In many ways I think Robin was a victim of the 2006 Evidence Act which allowed in all the hearsay evidence against him and since he and his daughter are dead, his reputation cannot be easily defended. When our lawmakers look back at what we should learn from the Bain case, I would hope they will review the disclosure rules so that the defence have the same obligations to disclose as the prosecution.

"And I think they need to consider whether families ought to be able to take defamation action against people who speak ill of their next of kin."

David Bain's lawyer, Michael Reed QC, last night labelled the documentary a "load of old rubbish" and "unadulterated rubbish".

"I don't know that Mr Bruce has spent one moment in the court. He has not referred to any of the forensic evidence, or any of the explanations given. And I am quite disgusted by the imbalance of the programme. I would be happy to debate the issue with Mr Bruce at any time. He hasn't bothered to ever contact the defence, or answer any of the crucial questions at all."

On the issue of Mr Young's evidence, Mr Reed said: "My understanding is that there are two people who can corroborate the fact that Mr Young did do the sale, and I'm not remotely interested in the nonsense that has been put forward."

Mr Reed said Mr Young's evidence was not hearsay, but direct evidence of what he saw and heard.

David Bain's chief supporter, Joe Karam, said claims made in promoting the documentary were "mischievous misrepresentations of the facts". "The programme itself perpetrates a fraud."

Mr Karam said letters were sent twice to Mr Bruce and TVNZ, inviting them to postpone the broadcast, or to speak to David Bain's lawyers for input into the programme. The documentary makers had sought David Bain directly.

Mr Karam said a letter sent to Mr Bruce and TVNZ gave a list of 14 items that David Bain's defence team said linked Robin Bain to the murders.

Robin Bain was portrayed in last night's documentary as "a missionary, a teacher for 37 years, a man with a wry sense of humour who loved his family".

Valerie Boyd, Margaret Bain's sister, said last night the documentary's questions needed to be addressed.

"We never believed the rubbish about Robin. And this is rebuilding his good reputation - it's long overdue. We just want the truth to be made public."

Meanwhile, a group campaigning against David Bain getting compensation for wrongful imprisonment believes last night's documentary could sway more people towards their cause.

Kent Parker, a spokesman for Justice for Robin Bain, said: "I think [the documentary] will definitely change public opinion. Obviously it doesn't change David Bain's legal status."

Mr Parker said 1620 people had signed and the goal was 10,000.

June 20, 1994: David Bain's parents Margaret and Robin Bain and siblings Arawa, Laniet and Stephen are found shot dead in their Dunedin home. Bain is subsequently charged with their murders.

May 1995: Bain, 23, is convicted and sentenced to a minimum 16 years.

May 2007: Privy Council quashes the convictions, after concluding "a substantial miscarriage of justice" had occurred. Bain is released from prison

March 2009: Bain is re-tried for the killings. He is acquitted after a three-month trial.

March 2010: It is revealed Bain's legal team could seek legal aid assistance for any compensation bid for his time behind bars.