David Bain supporters' challenge for a live, public debate has been accepted by a documentary maker - but only if Mr Bain shows up.
Journalist Bryan Bruce's documentary defending Mr Bain's dead father, Robin, aired on television this week.
The defence argued at the Bain retrial last year that Robin Bain had killed his family before shooting himself and that he had an incestuous relationship with his daughter Laniet.
David Bain was found not guilty of the murders.
The documentary questioned the defence's evidence, and police have begun an inquiry into the testimony of one witness.
Mr Bruce said yesterday he would take up a challenge to debate the case with Mr Bain's defence team if he appeared.
"I'm absolutely happy to debate them - provided David is there, because he's the only person who really hasn't spoken up about his father.
"And [he's] the only person who knows [what] the true relationship between Laniet and Robin was and what went on at that time."
Mr Bain's lawyer, Michael Reed, QC, who had earlier challenged Mr Bruce to a debate, said his client could not answer how the case had been handled.
"David Bain can't answer the questions about the trial ... This has nothing to do with David Bain.
"This is him [Mr Bruce] making allegations about how the trial was run. I'm the one who answers that.
"If he wants to debate with me publicly, on television, live, then we'll do it."
Mr Bruce said large issues were at stake, such as whether a dead man,not on trial, should be publicly accused "by hearsay" without getting a defence.
The 2006 Evidence Act had allowed one witness, Dean Cottle, to have his statement heard in 2008 despite being ruled out as an unreliable witness in the original trial, Mr Bruce said.
Mr Cottle's statement was read to the court last year when he missed his appearance because he had left the country, prompting police to issue an arrest warrant.
Daryl Young, the witness now under police scrutiny, was presented to the court at the last minute as a "surprise witness" - something the prosecution cannot do.
The New Zealand Law Society's criminal law subcommittee convener, Jonathan Kreb, said the defence could accuse other people because it was powerless to lay charges.
"It has to be remembered that the person that's most important in a criminal trial is, in fact, the accused."
If Mr Bruce's documentary had implied that David Bain was guilty, to avoid defamation it would have to prove his guilt or show it was a fair and honest opinion, Mr Kreb said.
The documentary's scrutiny of a witness could also be defamatory if it implied the witness had lied.