A businessman, convinced convicted double murderer Mark Lundy is innocent plans to put up a hefty reward for information leading to his release.

Geoff Levick, a horse breeder and former chemical company director from Kumeu in West Auckland, has spent three years investigating the case and is convinced Lundy did not murder his wife and daughter.

He told the Sunday Star-Times he had almost finished a book on the case and had given a draft appeal to Victoria University's Innocence Project, which had referred it to barristers for review.

Lundy's wife, Christine, 38, and daughter, Amber, 7, were killed with an axe at their Palmerston North home on August 29, 2000. The jury that found Lundy guilty in 2002 heard 160 witnesses in the six-week trial and deliberated for seven hours.

The Court of Appeal later threw out Lundy's appeal and increased his non-parole term from 17 years to 20 saying the evidence suggested Lundy planned his wife's killing carefully.

Mr Levick said he had the resources to put up a reward of more than $100,000, but had yet to settle on the amount.

"I'm not yet ready to announce how much the reward will be, I have to get it past my missus for a start."

He said years of studying transcripts and statements and his own inquiries had led him to believe that a P-addict connected to a businessman Lundy owed money to had been sent to the Lundy home to "teach him a lesson" and that matters "got out of hand".

The officer who led the inquiry, Detective Sergeant Ross Grantham, told the newspaper he was satisfied police got the right man.

Meanwhile, a former High Court judge who believes 20 people may be in jail wrongly says an independent tribunal to investigate such cases is unlikely.

Nearly two years ago Sir Thomas Thorp recommended a specialist tribunal be set up to investigate miscarriages of justice, but it has not happened.

He said New Zealand's review systems were insufficient and he believed there were more miscarriages of justice than acknowledged.