Police are going back through Arthur Easton's case file - looking for the person who killed him nearly 40 years ago.

Easton was stabbed in his Papakura home in 1985.

Alan Hall's wrongful murder conviction was only overturned after he'd spent 19 years in jail.

Police are now offering a $100,000 reward - until January 24 - for information leading to a conviction.

Detective Inspector Warrick Adkin says interviews are under way.

"As investigators, we have a responsibility to look at all options when solving crime, and today's announcement, we hope, will provide the financial incentive to those people who have not shared any information to come forward."

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