Arthur Allan Thomas will petition the Prime Minister to reopen the investigation into the Crewe murders.

Twice convicted of murdering Jeannette and Harvey Crewe but pardoned in 1979, Mr Thomas said New Zealanders - and the couple's only child, Rochelle Crewe - deserved to know who killed her parents.

"It's going to the Prime Minister. The police are just cover-uppers and the Prime Minister is the only man who can do something about it."

Mr Thomas' son-in-law, William Rowe, hoped prominent Kiwis who had called for a new inquiry would back an internet petition available at petitiononline.co.nz/petition/reopen-the-crewe-murder-case/20.

"The response from ordinary New Zealanders showed they want the case reopened, bugger the cost."