There is more than $700,000 of police rewards on offer in Tasmania and yet some of the state's most high-profile murders remain unsolved.

While rewards have proved successful in other states, experts have raised doubts as to whether any amount of money can overcome the family bonds in the island state.

It is two years since police offered $250,000 to solve the execution-style murder of Shane Barker in the small Tasmanian town of Campbell Town.

It is a record reward for Tasmania.

Police were convinced at time that they were just weeks away from cracking the 2009 murder.

But the identity of the person who lay in wait for the 36-year-old father, firing four bullets into him as he returned home, remains a mystery.

Shane Geoffrey Barker was shot at his Campbell Town home in 2009 but no one has been charged with his murder. ( Courtesy of the Mercury )

Acting Assistant-Commissioner Tony Cerritelli said despite the lack of results, the reward was a legitimate investigative tool.

"The investigation team at the time believed on pretty good grounds that it was good option to pursue," he said.

"It's a substantial amount of money and that's why it was put up."

Mr Barker's is not the only unsolved murder that detectives are offering big money for those willing to tell what they know.

There are more than $700,000 of police rewards on offer for some of the state's most high-profile murders (and suspected murders) and yet they remain unsolved.

"There's a whole range of motivations that people have in order to co-operate with police," Acting Assistant Commissioner Cerritelli said.

"One of those motivations is money and depending on where we are sitting with an investigation at any particular moment in time, the offer of money can be life-changing for many people.

"Our formula that we adopt here is similar to the methodology used across Australia and that is the type of reward you offer is going to be dependent on the facts of the case, the type of cohort that you're working with or against."

Police acknowledge that getting those who know to dob on potentially dangerous criminals is difficult across Australia.

Ivan Milat was caught in NSW after a substantial reward was offered.

But rewards have had some high-profile successes interstate, they include the 1996 conviction of notorious backpacker serial killer Ivan Milat after New South Wales Police posted a $500,000 reward and the arrest of crime boss Tony Mokbel after one of his gang turned snitch in return for a $1 million reward and immunity from prosecution.

While rewards have proved successful in other states, experts have raised doubts as to whether any amount of money can overcome the family bonds in this island state.

Forensic psychologist Dr Tessa Crawley has done a lot of work in prisons and in some of the state's smaller communities.

Dr Crawley believes Tasmania's "watery borders" have also created a barrier to speaking out.

"In a small community like Tasmania with such close-knit communities, such close-knit family networks, the fear of being identified as the informant, I can't image much change from a monetary perspective that can overcome that," Dr Crawley said.

"Small communities are very tight knit so there's a likelihood that when you speak out it's more likely that even if you are anonymous, it's going to be known that it was you who spoke out and that's a very real fear that people have.

"So no matter what the reward might be, their fear of danger potentially outstrips any monetary reward because in a small community like Tasmania, like some of our smaller towns, it's too difficult to hide and to protect your anonymity if you do speak up."

Those networks and the size of the island make informing police and moving away difficult.

Helen Munnings has not been seen since Wednesday afternoon, 23rd July 2008.

"The reality in Tasmania is you don't actually stay anonymous - you can be anywhere in any part of Tasmania and see somebody that you know or somebody who knows somebody that you know and I think most people realise that - that it's hard to be completely anonymous in Tasmania," Dr Crawley said.

She said many simply coud not escape the island.

"There's a percentage of Tasmanian people who've not spent a lot of time outside of Tasmania, there's a percentage of Tasmanian people who don't have family anywhere else and there's a percentage of Tasmanian people who are very intrinsically linked to their communities and to move them away, again the disincentive outstrips the reward," she said..

"And this I think is a problem that we keep coming up against when you think about rewards and behaviour - is the incentive higher than the disincentive?"

Dr Crawley said how crime rewards are structured needs to be reviewed - saying that to be effective, rewards have to be salient, meaningful and powerful enough to outweigh the disincentives to behaviour change.

"So when we talk about monetary rewards for policing - they're not immediate, they may not be meaningful — so for example if we're talking about wealthy people who have inside knowledge we assume that the people with the knowledge are desperate for that money but that might not be the case, that might

not be enough money to make a difference in that person's life," she said.

"So there are a number of barriers to those rewards actually working when we look at it from a psychological perspective."

But Tasmania Police maintain that rewards have an important place in investigations because they also refocus public attention on the crime and make people think about what they know.

"I wouldn't like to think that they can never be solved ... we're never going to let the families down ... we'll always be working on them and our aim is to bring the persons responsible to justice ... we'll never, ever give up," Acting Assistant Commissioner Cerritelli said.

But it is cold comfort to the parents of Tasmania's missing and murdered, some of whom have died waiting for answers.