So-called psychics on Sensing Murder were an "entertainment angle" to boost viewer numbers and detectives should focus on the benefit of prime-time exposure for cold cases, a researcher for the show told police.

The researcher told police "no matter what you think of the value (or otherwise) of psychics, Sensing Murder can be seen as a valuable opportunity to refresh attempts to get answers to some long-held unanswered questions".

"For my perspective, the presence of the psychics is more an entertainment angle aimed at drawing in viewers and the real strength of the programme is in the publicity that long-running investigations can get and, hopefully, new information is gleaned as a result," the researcher wrote.

Police disagreed with internal emails released through the Official Information Act revealing that not one show in the 39 episodes filmed over more than a decade had provided a single piece of useful information.

The top-rating Sensing Murder returned to TVNZ 2 this year with an average of 342,800 viewers watching each episode.

The show featured so-called psychics Kelvin Cruickshank, Deb Webber and Sue Nicholson, pitting them against three unsolved murders and the disappearance of a child. All are still unsolved.

It was made by television production company Screentime, which distanced itself from the comments of its researcher, backing the visions of the three stars of the show.

That support won't be echoed by police approached for help with the cold cases, with experienced detectives saying in emails it has never produced any useful information.

One detective sergeant said: "I wouldn't be happy to be involved in the programme and I don't think ... police should. The obvious issue is that these shows have never led to an investigation being progressed or arrest made as far as I am aware."

One detective inspector contemplating the benefit of high-profile exposure for cold cases said: "I accept [Sensing Murder] has never yet 'solved' or significantly contributed to any unsolved homicide clearance (that I'm aware of)."

In one email, police told Screentime it would not get involved with one of the cold-cases because doing so could harm police efforts.

"Given that this homicide remains unsolved any commentary may well compromise any further investigation in the future."

The official police policy on psychics says officers should not give any "credibility" to paranormal tips - and warned of the impact such information could have on those affected by crime.

It said hearing the "dreams and visions" of psychics "can cause distress to families and can give them an unrealistic sense of hope".

A spokesman for Police National Headquarters said any information received by police was assessed to see what relevance it had - and that media coverage of cases could generate new information.

"We are not aware however of any specific information arising from the Sensing Murderprogramme which has assisted an investigation."

The email trail showed police so keen to keep distance from the show they didn't want free-phone or Crimestoppers phone numbers listed at the end of episodes.

The closest police got to co-operating was allowing Screentime to use historical footage from one of its other shows, Police 10-7, in one episode of Sensing Murder.

Managing director of showmakers' Screentime Philly De Lacey offered another reason for the show - she said it gave comfort and closure to the families of those who had died.

"That came from the researcher. That's not the general view of the programme. I think the psychics are incredibly accurate."

She would not describe herself as a believer but said information coming from those portrayed as psychics "must be genuine" because she could not explain it.

Not only was there value in that information, but it gave cold cases "a chance for hour-long exposure on television".

"A lot of these people are fighting any which way they possibly can to get exposure for their cases. If you can assist with that in any way you can ... then I actually think that is a wonderful thing."

When told the show had prompted nothing in 39 episodes, de Lacey said there had been information of value.

"There may be reasons why that information may or may not be able to be taken forward."

She said reasons might include psychics coming up with a name but "there being no evidence to back it up".

De Lacey said the show also helped grieving families who found comfort in getting answers.

"It's all very easy to sit there and go, 'No, no, no - you shouldn't have psychics', but what if these people want it? What if it makes them feel better after years and years and years of misery?"

The Herald on Sunday asked each of the psychics for comment. Cruickshank declined and the others did not respond.

TVNZ's in-house lawyer Brent McAnulty responded to an OIA request about the show earlier this year, saying the "mediums do receive a fee for their appearance" but it wasn't paid by TVNZ.

"The mediums all believe that they have a gift that they can assist others with and there is no intention to deceive whatsoever," he said.

A TVNZ spokeswoman said the show was "compelling" for viewers and it wasn't for the broadcaster to tell people what they should believe.

"We accept that not all our programming will appeal to all people all the time. As a broadcaster our aim is to provide variety and balance as best we can."

Police detective-turned-defence lawyer Topny Bouchier said grieving families with no answers would "clutch at anything to get closure".

"In some ways, I think it is exploiting the vulnerable. Crime shows sell advertising whatever form they take and that's all broadcasters are interested in."

Former chair of NZ Skeptics Vicki Hyde said the show exploited grieving families for entertainment and ratings with the collusion of national media.

"Prime-time exposure of cold cases can help, but when it's framed in such an exploitative, manipulative way you have to seriously question whether the end justifies the means.

"The patent lack of results worldwide by this distasteful franchise suggests it should be consigned to the bin as neither entertaining nor effective.

"The show has always been about exploitainment, not about justice or acceptable policing process."