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.
In May a police officer left a file containing information about a sexual assault on the dashboard of an unmarked police car, with the victim's name and other personal details clearly visible through the window. The officer apologised after a member of the public alerted the Herald to the blunder.