Easton was killed after an intruder entered his Papakura home in Auckland nearly four decades ago.
Police announced the reward in October last year, which will remain in place until midnight on January 24.
It was offered for a limited period of three months to encourage people who have credible, current information to come forward now, and not leave it any longer, police said.
On Sunday, October 13 in 1985, a male intruder entered the Easton family home in Grove Rd at 8pm.
Police say Easton, 52, and his two teenage sons confronted this offender in the hallway, which led to a violent altercation and ultimately the death.
Detective Inspector Warrick Adkin said it was not too late to come forward and provide Easton's family with the answers they deserve.
"We cannot provide details while the investigation is ongoing, but we are committed to getting a result as soon as possible.
"We are determined to find the person responsible for killing Arthur and we know there are still people who have not yet spoken to police and who may have information about the events around Arthur's death."
Easton's death led to the wrongful conviction of Alan Hall, which was overturned in 2022.
Three people charged with perverting the course of justice during Hall's case will go on trial in 2026.
Easton's family gave their first public statement in December last year.
Son Patrick said the family knew there were people who had knowledge that could help with the case.
"We are here to ask for help, and to get justice for our father.
"He taught us to always do the right thing and he would have loved to see and be part of the lives of his eight grandchildren and soon-to-be five great-grandchildren."
Patrick said his father's death had "sucked a lot of joy out of our lives".
