A police officer should have acted differently after realising he'd left a loaded Glock pistol in a bathroom in Parliament, an investigation has concluded.

The gun was left in a bathroom accessible by the public for well over an hour.

Free guided tours run daily on the hour at Parliament, and schools often visit.

The Diplomatic Protection Service officer was on his way to Wellington Airport on work duties when he realised he had left the gun behind on June 16 last year.

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Police have concluded their investigation into the mistake and Assistant Commissioner Mike Rusbatch said it should have been dealt with differently.

"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," Rusbatch said.

"However, the loss should have been dealt with differently once it was discovered to minimise the risk to the public."

Police said when the officer realised his mistake, he immediately asked a colleague travelling with him to return to Parliament to recover the gun.

This second officer arrived at the bathroom only a few moments after the firearm was discovered by a member of the public, police said. The officer retrieved the firearm an hour and 25 minutes after the officer had realised it had been left behind.

The subsequent police investigation was partially focused on the steps taken to secure the firearm once its loss had been discovered.

The investigation accepted there was no intention to leave the firearm in the bathroom, but also looked at the options available to recover the firearm at the time.

Police would not reveal the outcome of an employment process with the officers, but said both continued to work in Protection Services.

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Rusbatch said "lessons have been learned" after the unprecedented incident.

The Diplomatic Protection Service (DPS) is a branch of police that has an office at Parliament, and is also responsible for the protection of the Prime Minister.

MPs were at Parliament on the day the gun was left there. Then Prime Minister John Key was in Auckland.

Key strongly backed the member of his protection squad, saying he was "the most professional guy I've dealt with".

Labour labelled the incident as "shabby" and could have been much more serious had the person who found the Glock firearm not acted responsibly.

Former police negotiator and detective Lance Burdett, who has VIP protection experience and now runs a private company dealing with negotiation skills, said at the time that a protection officer leaving a gun behind was the "ultimate sin".

"When you train you very nearly shower with it, it never leaves your side. If you get caught without it in training, the consequences are massive."

DPS officers would likely wear the weapon on a holster around their hips, and that was normally taken off to use the toilet, Burdett said.