Key Points:

EXCLUSIVE: Police have offered $800,000 in reward money in the past five years - but have not paid out a cent.

Officers said that the fact no money had been handed over to informants did not mean the policy was a failure, though one expert questioned the likelihood of it leading to arrests and some regarded rewards as no more than a way of getting easy media coverage for a case which the public had forgotten about.

Among the high profile cases to have had a reward attached are $75,000 in the case of Kayo Matsuzawa, who was murdered in Auckland in 1998, $50,000 over the disappearance of Grant Trevor Adams in Tauranga, and $300,000 for information leading to the return of military medals stolen from the Waiouru Army Museum in December.

The information was released to under the Official Information Act more than five months late by police, but only after a complaint was lodged with the Ombudsman when the police at first failed to reveal the full details.

Detective Senior Sergeant Stuart McGill said rewards were offered to put cases back into the minds of the public and give an incentive to people who may have heard something or know something.

Despite no money being paid out to people in the last five years, Mr McGill - based at the National Bureau of Investigation at national police headquarters in Wellington - said offering rewards did work. He said some of the cases had been solved.

"It's hard to determine what has caused information to come forward, whether it is the offering of the reward, the publicity around it or whether it is simply part of the investigation, coincidentally at the same sort of time around that people have come forward," Mr McGill said.

Greg Newbold, associate professor in sociology at the University of Canterbury, said people in the criminal world relied on good relationships with fellow-criminals and were not going to risk being ostracised from the community for the rest of their lives.

"It's the worst thing you can ever do. I'm sure someone knows who's got those medals but they're not going to come forward because of loyalty to the code probably," said Mr Newbold, who specialises in crime and criminal justice.

He said $50,000, the amount offered for information in some historical homicides, was only a year's wages.

"Narks do it but they have to move to a different part of the country or go on the police protection scheme and get relocated to other parts of Australasia. It's a pretty big step to take for a measly 50 grand," Mr Newbold said.

Senior lecturer of psychology at Victoria University of Wellington Dr Marc Wilson said there were several reasons why people with information might not contact the police.

He said no research had been carried out on police rewards in New Zealand but it was possible to speculate.

Dr Wilson said there were a variety of reasons why people might not come forward. He said they might not know they had valuable information or could be out of the country or dead.

But Dr Wilson also said a large reward could make people feel as though they were being bribed to give information.

"The larger the reward, the greater the possibility that people have to acknowledge to themselves that there is something bad going on or that they've done something wrong. We strive to avoid feeling bad about ourselves.

"And if you get pulled into a criminal case just by virtue of being associated, you probably want to forget it. You've got a psychological incentive to not be involved and in fact the larger the reward, the more likely you are to shy away from involvement because of what you have to admit to yourself," Dr Wilson said.

Dr Wilson said police benefited from offering rewards because rewards reinforced the public's belief that they were trying to do something.

"It doesn't matter what the case, the police probably get a lot of pressure from the public and people higher up the food chain to get results and this is a way for the police to do something," Dr Wilson said.

The offering of a reward could get the case back in the news, but Mr McGill said it would be too "simplistic" to say rewards were being offered just to get media attention.

Auckland University of Technology journalism school curriculum leader Martin Hirst said stories about police rewards "write themselves".

He said: "It's a newsworthy story, an unsolved murder. What the police do, when they announce they are going to give a reward, it gives a new take to the story and might give it more legs for a while."

"The cops aren't stupid, they know that," Dr Hirst said.

* An Official Information Act request was lodged by with police in June 2007. A partial response was received in August - over one month late. After a complaint to the Ombudsman, received the information this month, though police have said the letter was sent shortly before Christmas.

Police policies on offering rewards

The officer in charge makes an application to their District Commander. If approved the application is sent to the Police National Headquarters to make sure the wording of the reward is allowable under police policies and the Summary of Offences Act which governs the offering of rewards.

If there is immunity for prosecution included in the reward offer, it must also be sent to Crown Law.

The money for rewards comes from the police district where the reward is lodged.

If the reward money has been put up by the public, police remain in control of who gets the reward for what information given. Checks are also done to make sure the money is available and can be placed in a trust by police.

Rewards offered since 2002

$300,000 reward offered for information leading to the return of military medals stolen from the Waiouru Army Museum in December last year and a conviction of those responsible. The money was put up by British medals collector Lord Michael Ashcroft and an anonymous New Zealand businessman.

$75,000 reward offered for information leading to a prosecution in the murder case of Kayo Matsuzawa in 1998.

$50,000 reward offered in relation to the disappearance of Grant Trevor Adams who was last seen alive in Tauranga in November, 2005. Offer in force until December 31, 2007. Police have made an arrest in relation to Mr Adams' death. At the time police said the reward was not claimed.

$50,000 reward offered for information that closes the case in the disappearance of Joanne Chatfield who went missing on Princes St in Auckland in November 1988. The offer is in force until February this year.

$50,000 reward offered in relation to the disappearance of Margaret Kaye Stewart who went missing in June, 2005 in the Rimutaka Forest Park in Wainuiomata. In force until July 13, 2007.

$50,000 reward offered in relation to the disappearance of Nicholas Pike who was last seen alive in March 2002 in Mt Mauganui. In force until July, 2007.

$50,000 reward offered in relation to the death of Katrina Jeffries whose body was found in July 2005 in the Waikowhai Reserve in West Auckland. In force until December 31, 2007.

$20,000 reward offered for information in relation to finding two offenders for the alleged attempted murder and sexual violation of a 14 year-old girl. In force until September 2005.

$20,000 reward offered for the apprehension of Joe Tua Coleman who was wanted in connection with the murder of George Matahaere in force until February 2003.

$20,000 for evidence or information that leads to the location of Sara Louise Neithe or her body and the conviction of any person responsible and information that results to the location of her car. In force until December 2003. The reward was offered again after no one came forward, this time with an offer of $50,000.

$15,000 reward offered for information leading to the conviction of a person responsible for the arson of a house in Prembleton, Christchurch in 2002. The reward was a joint offer by police and Farmers Mutual Group. In force until March 2003.

$13,000 reward for information leading to the arrest of Nai Yin Xue who is accused of murdering his wife in September 2007.

$5,000 reward offered for material information or evidence that leads to the apprehension of Michael Joseph Cavanagh and Shannon Kay Stevens. In force until July, 2004.