The officer who left his loaded gun in a bathroom at Parliament has been with Prime Minister John Key's protection team for eight years.
Mr Key said today it was a catastrophic error and the officer would be mortified.
Police would not say whether the gun, found on Friday, was discovered by a parliamentary staffer, an MP or a member of the public.
The prime minister said he still backed the officer who made the mistake.
"He's utterly professional. I trust him implicitly, and he's just made an awful error," he said.
"I know it's a terrible headline, and he will be beating himself up enormously, but he has just genuinely done that - made an error."
Mr Key wouldn't say if the officer was still working on his protection team.
The police and Parliamentary Service have been holding their own internal inquiries.