Police have been left red faced after an absent-minded cop left a loaded rifle magazine on top of his police car and drove away.

Inspector Matt Broderick, acting area commander for Hawke's Bay, said the M4 Bushmaster rifle magazine was lost in Napier today in what he called a "regrettable" error."

Napier Police were executing a search warrant at an address in Trinity Crescent, Napier and during the initial stages of the operation police were carrying firearms," he said.

"At the conclusion of the search warrant an officer inadvertently placed the rifle magazine on the roof of his patrol car and drove towards Hastings. The magazine, containing ammunition, has fallen from the car roof.

"Police are asking that should anybody find the magazine that they return it to the nearest police station. Alternatively they can call 111 and local Police will recover it.

The magazine is described as curved black plastic shape containing ammunition.

"The error is regrettable and should not have occurred," Broderick said.

"As is commonplace following incidents like this, police have begun a policy, practice and procedure review of the incident."

It comes after an incident at Parliament last year when a Diplomatic Protection Service officer left a loaded Glock pistol in a bathroom that was accessible to the public for well over an hour. The officer was on his way to Wellington Airport on work duties when he realised he'd left his gun behind on June 16 last year.

A police review of the incident found the officer should have dealt with the situation differently once he realised his mistake.

"We are in no doubt about the potential risk. Our staff are human and we accept that the firearm being left in the bathroom was a genuine mistake," Assistant Commissioner Mike Rusbatch said earlier this month.

-NZ Herald