A police officer who left a loaded gun in a bathroom at Parliament will doing "a lot of soul-searching" and could face up to three investigations, a security specialist once responsible for guarding prime ministers says.

Police are investigating how a loaded Glock pistol was left behind in a Parliament bathroom on Thursday.

The gun was recovered by police after they became aware it was there.

ROSS GIBLIN Labour police spokesman Stuart Nash does not believe the police officer who left their loaded gun in a Parliament bathroom should be fired.

In a statement, police said: "As soon as police were made aware that the firearm had been left in the bathroom, staff attended and recovered it from the occupant, and we would like to thank them for their assistance.

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"This was a regrettable incident that we are taking seriously, and we acknowledge the potential risk that this could have posed."

The statement says police have started an investigation into the incident, and would not comment further while they were looking into the matter.

Police have not confirmed who owned the firearm, but police staff most commonly seen around Parliament are members of the Prime Minister's diplomatic protection service, who are routinely armed.

A spokeswoman for Prime Minister John Key declined to comment, saying it was a matter for police.

'THE WORST THING YOU CAN DO'

Security specialist Lance Burdett, who used to run an Auckland-based VIP protection squad responsible for looking after prime ministers, said the breach was "the worst thing you can do, absolutely".

"He or she is highly trained, and that's the first thing you train is never let it [your gun] out of your sight.

"Out on the course, you wear it the whole time... it is never to leave your side so you get used to not leaving it behind and always having it with you."

Burdett said firearm holsters were often worn on the hip, and it had likely been taken off to use the toilet, then left behind out of "absent-mindedness".

MULTIPLE INVESTIGATIONS LIKELY

"To a member of the public, that sounds stupid...but how many times have you left your keys or cellphone somewhere?

"Yes it's dumb, it should never happen, but these things do happen."

Burdett said there could be up to three investigations following the breach: internal, criminal, and through the Independent Police Conduct Authority.

If the staff member was part of the DPS, they would almost certainly lose their job, while they could face dismissal from the police force entirely.

They would be doing "a lot of soul-searching" while the investigations were underway, Burdett said.

"Whatever repercussions are for the person that's left it there, they themselves will be putting themselves through more hoops than anyone else ever could."

SACKING NOT NEEDED - LABOUR

Labour's police spokesman Stuart Nash didn't want to see the person responsible fired, instead a written warning should be issued.

"It's a serious breach but this person is never going to leave their firearm alone again and nothing bad has come from it."

Nash said he'd hate to see Parliament and MP security tightened as a result of the incident.

"One of the wonderful things about New Zealand MPs is we're incredibly accessible and I'd hate to see that change."

While nothing bad happened, Nash said it could have been much worse given there's "tight security getting into Parliament but no security to get out".

PARLIAMENTARY SERVICE TALKING TO POLICE

Parliamentary Service communications manager Kathy Milne said security were in conversations with the police to "uncover" how it happened.

"We're just going to let the police do their bit," she said, referring to it as a police matter.

Milne said the Parliamentary Service would have to carry out a review of the incident.

The incident comes after a protester was able to drive a car loaded with fuel up to the front of Parliament, leading police to lock down the precinct for several hours on the day of the Budget.