The handling of David Bain's murder case was so "deficient" and "unfair" it left New Zealand's reputation with a "black eye", former Canadian Supreme Court Justice Ian Binnie says.
In 2012, Justice Binnie recommended Mr Bain receive compensation for the 13 years he spent behind bars before his 1995 conviction for murdering his parents and three siblings was overturned.
His report was rejected by then Justice Minister Judith Collins after a peer review and a subsequent report by another former judge concluded compensation was not merited.
Last year the Government agreed to make an ex-gratia payment of $925,000 to Mr Bain to end the long-running claim.
On Saturday, Justice Binnie told Radio New Zealand his report and the second were judged by different standards despite using the same approach.
"Why is it that when the application of the test runs in favour of Bain, the Government denies it and when it runs against Bain, the Government ignores it," he said.
"There's a double standard and it all comes out against David Bain."
New Zealand needed to create institutions to make sure cases such as Mr Bain's were kept away from politics, he said.
Justice Binnie said regardless of whether people thought Mr Bain was innocent or not, the handling of the case had not been "fair play".
"I don't think there's been a case in my career where I feel the process has been so deficient and operated so unfairly, to the extent the case is known now outside of New Zealand," he said.
"It's given New Zealand a black eye."
Mr Bain was convicted in May, 1995, of the five murders in Dunedin 11 months earlier.
The Privy Council quashed his convictions and he was acquitted in a retrial in 2009.