The journalist whose podcast details the horrific Bain murders has spoken to Newstalk ZB about why he came to believe in David Bain's guilt.

Reporter Martin Van Beynen, who covered the 12-week retrial in 2009 and subsequently wrote a book about it and published a column stating the jury got it wrong, said the key piece of evidence was the "terrible, vicious, desperate" fight between the killer and Bain's brother Stephen.

"The equation is either Robin or David," Van Beynen told Newstalk ZB's Andrew Dickens.

"You've got to say, at first glance anyway, David with the blood on his clothes, the injuries and so forth he looks like the more likely suspect. But there's a lot more to it.

"The glasses found in David's room, they are damaged and the lens was found in Stephen's room."

The fight took Stephen and the killer tumbling over the floor and the bed in a physical brawl, which ended with Stephen being strangled by his T-shirt. Bain's father Robin was seen as being quite frail while 22-year-old Bain was healthy.

"Robin was regarded as being quite frail but that doesn't mean to say he couldn't do it.

"With adrenaline and rage you could possibly imagine even a person not very fit or strong doing it.

"There are no silver bullets in this case. There's nothing you can say 'well this is the absolute proof'. You have to put it all together."

Van Beynen, a Christchurch Press reporter, has released a 10-part podcast on the Bain murders this week.

It explores the case from start to finish, picking through evidence, the mysteries and motives, and interviewing fresh witnesses.

The reporter aimed to finally answer who the killer was.

Bain was first found guilty of the murder of his family - Robin, 58, Margaret, 50, Arawa, 19, Laniet, 18 and Stephen, 14 - in May 1995 and jailed for life with a minimum term of 16 years. They were fatally shot on June 20, 1994, in their Dunedin home.

After years of legal battles, he was granted a retrial in 2007. He was acquitted at the 2009 trial.

Despite having an opinion Van Beynen tried to be objective and tackle the story from a "historical approach".

He acknowledged the problems in the evidence brought against Bain and criticisms of the police investigation.

"My credibility is at stake. I wasn't going to make it a work of advocacy. I tried to be fair."

Every podcast episode starts with Bain's 111 call.

The paramedic who took the call said he'd never come across anyone who had that level of trauma who could provide detail so clear and crisply, Van Beynen said on Newstalk ZB.

"No one knows how people react to these things. Who's to say David, given his makeup, wasn't genuine in the way he rang 111?

"There's a lot more to it than just blood."

Last year the Government agreed to make an ex gratia payment to Bain in the interests of bringing closure to the long-running claim. A full and final payment of $925,000 was accepted by Bain's team.

He has now changed his name from David Cullen Bain to William David Cullen Davies, taking his wife Liz's surname.

It is believed the couple, who have a young child, have moved to Australia to start afresh.