Leaving a gun lying around is "the worst thing you could do", a former head of the police VIP protection squad has said after a pistol was found in a Parliament bathroom.

The loaded Glock sidearm was found in a disabled bathroom on Thursday by a member of the public. It belonged to a member of the Diplomatic Protection Service and was handed in without incident.

The DPS is a branch of police that has an office at parliament, and is also responsible for the protection of the Prime Minister.

Lance Burdett, former police negotiator, detective, and head of the VIP protection squad said that officers are trained to have their firearms on them at all times if required.

"During training it becomes a part of you and yes, leaving it anywhere is basically the worst thing you could do," he said.

"The policy there's quite clear, you and your firearm never part company."

Burdett said the police staffer in question will be facing a serious investigation and will have learnt their lesson.

Labour's police spokesperson Stuart Nash believes it was a serious incident, but the staffer's mistake shouldn't be punished too harshly

"I think the guy should receive a written warning, I don't think you can do anything less than that, but I don't think he should lose his job."

Nash believes the unattended gun was simply a matter of forgetfulness.

"The guy went to the toilet, he took his gun off his belt, he dropped his trousers, he went toilet, did them up again and forgot to put the gun back on his belt."

Police said they will be investigating.