A new suspect has been named as the murderer of Harvey and Jeanette Crewe - a detective who played a pivotal role in the police investigation into their deaths.

Detective Sergeant Len Johnston has been accused by author Ian Wishart of being the double murderer behind New Zealand's greatest unsolved murder mystery.

Arthur Allan Thomas: The Inside Story hits bookshops tomorrow - but has already attracted a storm of outrage from those who knew Johnston, who died in 1978.

It has also sparked a bitter attack on Wishart from Police Association president Greg O'Connor.

The Crewes disappeared from their blood-spattered Waikato home in June 1970. Their daughter Rochelle, 2, was found crying in her cot five days after the Crewes were last seen.

Jeanette Crewe's body was found in August that year and Harvey Crewe's in October. Jeanette Crewe had been badly beaten, suffering a broken jaw. Both Crewes had been shot to death.

Farmer Arthur Allan Thomas was convicted of murder, but was pardoned in 1979 after mounting public protest and the personal intervention of Prime Minister Rob Muldoon.

A Royal Commission set up to investigate the case found inquiry head Detective Inspector Bruce Hutton and Johnston had planted a rifle shell casing at the Crewe house to implicate Thomas.

The Herald on Sunday has obtained a copy of the book, in which Wishart claims Johnston was a "dirty cop" who was "physically violent and prone to making death threats".

He said Johnston was known as "The Fitter" among colleagues for his ability to arrange evidence to suggest guilt.

Other details cited as evidence by Wishart include the claim that Johnston was behind an arson at a police station with the intent of intimidating fellow officers.

Wishart also said that Johnston matched the US Federal Bureau of Investigation profile of an "anti-social personality".

The book also draws on links between Johnston and the Crewes before the murders.

Wishart said that Johnston investigated a burglary at the couple's house in 1967 and "would have known the layout of the house".

It was also "highly likely" he asked Jeanette Crewe the location of the spare key.

Wishart said it raised questions as to whether Johnston was behind arsons at the Crewe house in 1968 and 1969 and whether he was "stalking Jeanette".

Wishart also offered the theory that the murder could have come after a threat from Jeanette Crewe to report Johnston's behaviour to her husband.

Wishart also asks: "Did he rape her then execute her afterwards?"

One of the great mysteries of the Crewe murders was Rochelle Crewe, aged 2 and found crying in her cot when the bloodstained scene was discovered.

Doctors who examined her said she could not have been abandoned for five full days since the murders. Witnesses reported seeing a woman at the house but she was never identified.

Wishart said Johnston would have had a "female criminal acquaintance over whom he exercised power", possibly a prostitute.

"She may ultimately have been killed by Johnston to maintain her silence."

Blackmail was offered by Wishart as an alternative motive for Johnston.

He said the 1967 burglary was "fishy" and questioned whether Johnston was blackmailing the Crewes over trying to cheat their insurance.

Then, the night before the couple's wedding anniversary, "something snaps", Wishart wrote.

In a chapter called "The New Prime Suspect", Wishart writes that Johnston "confronts the couple and kills them to shut them up and protect his own career and reputation". Wishart said that Johnston's knowledge as a detective would have allowed him to "create the perfect crime scene".

Wishart said it was significant that Johnston was also closely linked to the axle used to weigh Harvey Crewe's body down in the river. The axle tied Thomas to the case - after Johnston found axle stubs in a tip on the farmer's land.

The tip - like the lawn where the "planted" cartridge case was found - had already been searched by police and nothing found.

Bruce Hutton, now 82, was incredulous when told of Wishart's claim. "Johnston was a detective. He killed the Crewes? A detective from the police killed the Crewes? I've never heard anything like it."

The police association's O'Connor said he would not read the book. "I won't encourage scurrilous rubbish. You've got to ask what this guy's on.

"I thought the guy (Wishart) was meant to be a Christian."

Arthur Allan Thomas: The Inside Story goes on sale tomorrow for $39.95.