Key Points:

Joe Karam has stopped counting the personal financial cost of his battle to free convicted murderer David Bain, but he estimates it is "likely to be millions".

A former All Black, Mr Karam first became interested in Bain in 1996 after reading a newspaper article about supporters trying to raise money to get his case to the Privy Council.

"It struck me that they must have been genuinely convinced about what they were doing to go to such lengths, when this was just a person they had only known for a couple of years."

The Bain family had been in Papua New Guinea, only moving to Dunedin in 1990, Mr Karam said.

He said he had planned to check if the supporters were genuine, then send them a couple of hundred dollars for their kitty.

"And one thing led to another after that."

In 2000, a newspaper reported Mr Karam had spent $750,000 on his quest to free Bain, who he believed was wrongfully convicted, but that figure no longer stands.

"I've probably been through the most productive earning years of a businessman's life, really - from early to mid 40s to mid to late 50s ... not getting any money.

"The real cost is obviously millions."

He described his crusade as "a very sapping exercise".

Only recently has he been able to get involved in business again, finally managing to "keep the wolf from the door", he said.

Mr Karam believed "without any shadow of doubt" that Bain was innocent.

There was not one defining moment when he believed Bain was innocent, but rather the evidence started to unravel, he said.

"Mainly because the police in particular were so obstructive in assisting with investigating things that didn't get investigated properly in the start."

Mr Karam has been vocal in his criticism of the police, alleging incompetence which resulted in the police suing him, unsuccessfully, for defamation.

Campaigning for Bain has taken up nearly all of Mr Karam's life since he became involved, but he only ever contemplated giving up when he was being sued.

Having written three books on Bain's case, he joked that his role on Bain's legal team was as "information bureau".

"There have been a vast amount of hearings and inquiries and reports, all of which I've been involved in."

Mr Karam travels from Auckland to visit Bain most months.

He said Bain spends his days working long hours in the administration office of Christchurch Prison's engineering workshop doing purchasing, administration and design, as well as studying computer-aided design.

Bain was "just a completely shattered individual" when Mr Karam first met him, but time has proven to be a good healer. "To that extent he isn't fraught with the total feeling of despair and destitution that he was when I met him.

"He has found a way to deal with prison life and he has applied himself to studying so that when he gets out of prison he'll be able to survive or make a life for himself."

Bain would like to stay in New Zealand in the future and make a career in computer-assisted design.

Mr Karam said Bain was highly regarded by the prison staff and had been a good influence on other prisoners.

"He's quite firm but very fair, he's gently natured but he won't be pushed around, he's very strong on principles."

Bain's extended family "have wiped him", Mr Karam said.

"They gave an ultimatum about 10 years ago that until he asked for God's forgiveness for having killed his family and admitted to everyone that that's what he had done they didn't want any more to do with him ...

"He wrote back and said, 'I won't do that because I didn't kill anybody'."

Bain was very close to his grandmother, who died last year in Otaki, but the family did not want him at the funeral.

Mr Karam travelled to London in March to assist Bain's Auckland barrister, Michael Reed, QC, during the five-day appeal hearing by the Privy Council's judicial committee.