A Rotorua stream is being scoured by police for evidence relating to the death of a security guard during a botched Rotorua bank robbery almost 36 years ago.

Rotorua police announced on Tuesday they were casting "fresh eyes" over Howard Percy's murder at the satellite branch of the ANZ in Rotorua.

The police dive squad, police search and rescue personnel, and land search and rescue volunteers began searching a 1km stretch of the Mangakakhi Stream and its banks on Friday afternoon.

Detective Inspector Tim Anderson said police are looking for a firearm, believed to be a .22 sawn-off rifle.

"We have received information from a reliable source that the weapon thrown into this stream after the robbery," he said.

"I am not prepared to say when we received this information or from who but I can say that reliable information we received prompted us to escalate the investigation."

It's not clear at this stage how long the search will take.

Mr Percy, 65, died on the way to hospital after being shot twice during the robbery of the bank on November 5, 1976.

He was shot after tackling the robber during the heist.

The offender escaped and police never caught him.

Earlier this week, Det Insp Anderson confirmed advances in forensic science in areas such as DNA analysis had given new impetus to the so-called cold case.