Detective Inspector Mark Loper, of Bay of Plenty Police, said upgraded charge was a result of further police inquires, including the analysis of post-mortem results.
Mr Loper said Mr Tunnicliffe's family had been advised of the murder charge and police would continue to work closely with the family to keep them informed.
Residents of his tiny settlement of Tokaanu, near Turangi, told the New Zealand Herald of their shock after the incident occurred.
His friend Andrew Blake said he was left "shocked and speechless" at the attack. He described Mr Tunnicliffe, who had been receiving a benefit for an injury, as a kind-hearted man who loved company and took in two dogs to look after.
"Everyone likes him ... He's a hell of a nice guy and loves helping people. Picking up and helping a hitchhiker would be totally in character for him - he's very trusting."