A former police officer and MP says he was pressured by the police commissioner of the day to change important evidence to the Crewe Royal Commission to suit the police case.

Ross Meurant told the Weekend Herald that on the morning he gave evidence to the 1980 commission, Police Commissioner Bob Walton tried to persuade him he had not thoroughly searched the Crewes' garden.

Mr Walton's "clear message was that I had been careless and that is why the [shellcase] hadn't been found".

Mr Meurant, a constable at the time, did the first search of a patch of garden where police claimed a subsequent search found a shellcase from Arthur Allan Thomas' rifle.

The royal commission concluded that in "an unspeakable outrage", Detective Inspector Bruce Hutton and Detective Len Johnston (who died in 1978) had planted the shellcase to frame Mr Thomas.

Mr Meurant testified at the Thomas trials that he had been thorough in his search of the garden, evidence that was an impediment to the police case that Mr Thomas had murdered Jeannette and Harvey Crewe.

Mr Meurant said he was a senior sergeant working in the Auckland police control room when he was called into the special operations room by Mr Walton and a senior Auckland police executive officer, Brian Wilkinson.

"Walton began asking me what I was going to say in my evidence," Mr Meurant said. "His clear message to me was that I had been careless and that is why the [shellcase] hadn't been found [in the initial search]."

Mr Meurant said he told the commissioner that the Crewe inquiry was his first homicide, he had scrupulously followed search procedure, he had been supervised and that he had been careful and methodical.

"I told him I had done the search on my hands and knees.

"He said, 'C'mon, detectives don't get down on their hands and knees and sift through dirt', or words to that effect.

"He was saying that detectives weren't as thorough as I said I was. I was stunned ..."

Mr Meurant said he had done a thorough grid search using a method set out by Mr Walton in a manual he had written for detectives.

"Here I was, locked into this investigation, I was about to get commissioned, I had a mortgage, a wife and kids, and I get this pressure."

Mr Meurant said the duty inspector, Phil Keber, saw him come out of the meeting.

"Phil said to me, 'Geez, you're white, Meurant, what happened there?' and when I told him, he was as disgusted as me."

Mr Keber told the Weekend Herald he had no recollection of the matter Mr Meurant describes. Mr Walton and Mr Wilkinson have died.

Mr Meurant said: "I realise that speaking ill of the dead is not good form, but I am not making this up."

The royal commission's report appears to bear out that Mr Meurant testified his search was thorough, and also his recollection that some police officers changed their evidence.

The report said: "The evidence of Mr Hutton and Mr [Murray] Jefferies particularly ... demonstrated a tendency to denigrate the thoroughness and care of the officers carrying out the search.

"The object of this evidence was obviously to establish that it is not very surprising that exhibit 350 [the Thomas shellcase] was not found during the ... [first] search.

"We find it unacceptable that the police should now say their own investigation was casual and slip-shod, although we can understand they are anxious to avoid the conclusion that exhibit 350 was planted."

Mr Meurant said that about the time he gave evidence, the commission received the ballistics report which concluded that the lead in the bodies could not have come from the shellcase police claimed was found.

"That was it - game, set and match," Mr Meurant said.

Mr Hutton and Mr Johnston denied having planted the shellcase and the Solicitor-General at the time, Paul Neazor, QC, decided there was insufficient evidence to charge them.

The Crewes' only child, Rochelle, has criticised the Neazor ruling, and the police decision not to reopen the case. A senior officer has instead been assigned to review the file.

Mr Meurant acknowledges being involved in suspect behaviour himself as a policeman in the 1970s and 1980s, but he now campaigns against such practices and is helping Ms Crewe in her efforts to have the murders reinvestigated.