A television investigative journalist says he has identified a potential third suspect in the infamous 42-year-old Crewe murders, and claims police won't re-investigate the case because of self interest.

Bryan Bruce says the information will be revealed in the screening of The Investigator on Tuesday night.

In what is one of New Zealand's most notorious unsolved killings, Jeanette and Harvey Crewe were shot dead at rural Pukekawa, south of Auckland, in June 1970.

Neighbour Arthur Allan Thomas was twice tried and convicted of the murders, but pardoned and freed in 1979 when it was revealed police had planted a cartridge case to frame him.

The suspicion then switched to Mrs Crewe's father Len Demler, who has since died, but Mr Bruce says his inquiries have led him to believe a third person was the killer.

He also says a police internal review of their investigation - launched in 2010 - is being used to deny him archive documents, which have been available to other journalists.

"I've learned from experience that when the cops make it difficult for me then I'm onto something they really don't want the public to know," he said in publicity material.

He said that from documents he did get his hands on, he would reveal "the shameful fact that for 42 years a chain of police commissioners have put the self interest of the police ahead of their duty to properly re-investigate this case".

Meanwhile, the Crewes' daughter Rochelle Crewe has told the New Zealand Herald police had denied her full access to her parents' homicide file, and she might not receive a copy of the completed review.

"Why spend money on a review if only the police themselves get to see it?"

Assistant Commissioner Allan Boreham said it was not appropriate to partially release any further information into the public domain.