The prime minister says the government's candidate in Longman made an innocent mistake when he claimed to have a prestigious military medal.

Trevor Ruthenberg has been forced to apologise to voters in the crucial Queensland seat after he falsely claimed to have a medal for distinguished service in peacekeeping and non-war operations.

He hopes voters will believe that he made a simple mistake in a biography he wrote about himself back in 2012.

He says he does have a lesser medal, recognising his service in the air force, and he simply wrote down the wrong medal name.

"I'm not a military imposter. I did serve in the military," Mr Ruthenberg has told ABC radio.

"I'm not claiming I had two medals. I only had a single medal. I'm not sure why I wrote what I wrote, but I did and I'm making steps to make that right."

Prime Minister Malcolm Turnbull said the candidate had called him to apologise for the embarrassing error.

"He's very proud of his service in the air force ... so he feels bad about it. I've spoken to him about this morning and I'm satisfied it's an honest mistake," Mr Turnbull told the ABC.

AAP

Mr Ruthenberg said he is solely responsible for an error in his Queensland parliamentary biography, penned shortly after he was elected as part of Campbell Newman's government.

The biography says he holds the Australian Service Medal, which is presented by the Governor-General on the recommendation of the Chief of the Defence Force.

In fact, he holds the Australian Defence Medal, which merely recognises four years of service.

The candidate said he couldn't explain why the error was made, but he'd asked the clerk of parliament to correct it.

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"It happened some time ago. Maybe I was tired, maybe I was rushed," he said.

"All I can do is apologise to folks if they feel they were misled."

Mr Ruthenberg is trying to win Longman for the government, after Labor's Susan Lamb was forced to quit the parliament over the dual-citizenship saga. She is recontesting the seat.