The lawyer for Arthur Allan Thomas believes the death of a key police figure in the Crewe murder investigation could bring more information on their deaths to light.
Bruce Hutton, 84, died in Auckland's Middlemore Hospital on Sunday night.
The former detective chief inspector was blamed for planting evidence that helped to convict Mr Thomas of the deaths of Jeanette and Harvey Crewe in 1971.
Mr Thomas spent nine years in prison for murdering the Crewes before being given a royal pardon in 1979 and released.
Peter Williams, who represented him at a Royal Commission inquiry, says people close to the investigation have withheld important information for more than 40 years.
Mr Williams says he hopes they'll now reveal that information now the last remaining lead investigator on the case has died.
"I think a lot of people were scared of the police and there's been a very determined group of police officers who out of so-called loyalty to Hutton have been very vigorous in their defence of Hutton."
Mr Williams says there is a possibility that people who would have been scared of reprisals may now come forward.
He told Radio New Zealand's Morning Report programme that the 1970s were a dark period in the history of the police.
Mr Williams is calling on police to release the findings of a review that has been under way since 2010. The police have declined to say when it will be completed.