The judge in Bain's trial 14 years ago decided not to allow evidence about Robin Bain's alleged incestuous relationship with his daughter Laniet, as he ruled it was hearsay.
"Now the Evidence Act 2006 changes all of that, allows it in, and I think a judge would have allowed it in anyway because after the last trial the Court of Appeal allowed that evidence to be publicised -- so it had to go in one way or the other," Michael Guest told Radio New Zealand.
"Now whether that tipped the balance or not I don't know, but it was still pretty powerful evidence which raised a motive against the father."
Mr Guest said he did not personally believe it, and was sad that it had been used, but the evidence raised a reasonable doubt in the mind of the jurors.
"I believe that any question of fact like that should be heard by a jury, and so I think there was an injustice that that evidence didn't go before the first trial," he said.
"But, having said that, I actually don't believe -- having looked at all the new evidence -- that in fact Laniet was telling the truth."
Laniet's friend Dean Cottle gave a statement to police after the killings, but before Bain was arrested, that she had told him her father had been having sex with her for years, and it was still going on.
Now the jury had returned its verdict, the matter should be put to rest, Mr Guest said.
"We've got a robust system that breaks down sometimes, but there has been a finding of not guilty -- that doesn't necessarily mean innocence, but he's entitled to that inference.
"He has been found not guilty after all this time by a jury that considered a hell of a lot of evidence, and he's entitled to that finding and he's entitled for the matter to be put to rest."
