Tasmania Police revealed on Monday morning the increased reward would be on offer for information relating to a conviction in each of the outstanding cases.

Some of the cases had offers of $30,000, while others had already been increased to $50,000 and $250,000.

But now, all seven investigations came with a $500,000 incentive for witnesses.

Those cases included Christopher Dean Watkins, Nancy Grunwaldt, Victoria Cafasso, Paul Winston Byrne, Helen Munnings, Simon Crisp, and Eve Askew.

Mr Watkins vanished from Mayfield in 2013, and police believe he was murdered.

His mother, Lillian Watkins, said the reward increase left her feeling "over the moon".

Knowing "in her heart" her son was murdered, she said finding the person responsible would give her family closure.

She described the past eight years as "hell" for her family, and said she even feared for her own life knowing the killer was still at large.

"There has been times I thought 'are they going to come for us', it's been difficult," she said.

IN OTHER NEWS:

Launceston-based Detective Inspector Craig Fox said the Watkins case had never been closed, and was "very close to being solvable".

Hopeful the reward increase would encourage witnesses to come forward, Detective Inspector Fox said police would revisit old leads, and re-interrogate previous suspects.

Witness statements early on in the investigation suggested Mr Watkins was taken from a unit at Box Street, driven to bushland, shot and buried.

But his body was never found.

While police had significant information relating to the case, Detective Inspector Fox said their focus would be to turn that information into strong evidence that could be used in court.

"We will investigate every avenue we can to bring justice for Chris," Detective Inspector Fox said.

Making the reward announcement on Monday, Tasmania Police Assistant Commissioner Jonathan Higgins said offering more money could give witnesses "a fresh start".

"In the passage of time someone's circumstances and allegiances may have changed and the offer of a life-changing amount of money may be the motivation they need to come forward," he said.

"If you have information about any of these cases, please come forward and help us provide some kind of closure to the families and loved ones of the victims. Families always deserve answers to what happened to their loved ones and we owe it to them to find those answers."

Paul Byrne was reported missing in 1996.

It was strongly suspected he was murdered in Rossarden after he was drinking at a pub with two men.

Days after he disappeared, the house he was last seen in was burned to the ground.

Paul's original reward was $100,000.

Nancy Grunwaldt went missing from the East Coast in 1993, and a reward of $30,000 was originally on offer.

The German backpacker had arrived in Tasmania a few days earlier and left her pack with the bike hirer, telling him she planned to ride down the East Coast to Hobart.

On March 11 she made a phone call to her parents and travelled by bus to St Helens where she spent the night at a hostel.

That phone call was the last time they heard from their daughter.

Victoria Cafasso was murdered on Beaumaris Beach on the East Coast in 1995, five days after arriving in Tasmania.

She had been stabbed 17 times and repeatedly struck with a blunt object.

No one has been charged despite several lengthy investigations.

Victoria's original reward was $50,000.

Eve Askew was just 14 when she was reported missing from her home in Fitzgerald in southern Tasmania in November 1991.

She has never been located and it's believed she met with foul play.

Eve's original reward was $100,000.

Young mother Helen Munnings disappeared from the Burnie area around July 23, 2008.

A Coroner determined Helen died in Burnie on or about this day, but could not determine how she died.

Police strongly suspect she was murdered, and a $250,000 reward was offered.

Simon Crisp was fatally shot as he put rubbish out at the Marrawah Tavern in the early hours of July 13, 2013.

After extensive investigations, no one has been charged.

Simon's original reward was $50,000.

Anyone with information relating to the cases should contact Crime Stoppers on 1800 333 000, or Tasmania Police directly on 131 444.