The LNP's candidate in the Longman by-election, Trevor Ruthenberg, has apologised for wrongly claiming he earned a prestigious defence service medal.

Mr Ruthenberg, the former state MP for the seat of Kallangur, listed the Australian Service Medal as one of his accolades on the official Queensland Parliament website, despite never receiving the distinction.

He received the Australian Defence Medal, which is awarded to those who have served for more than four years in the Australian Defence Force (ADF), as opposed to active service.

The LNP selected Mr Ruthenberg in May to contest the marginal seat of Longman against former Labor MP Susan Lamb, who quit Federal Parliament over the dual citizenship saga.

The LNP selected Mr Ruthenberg in May to contest the marginal seat of Longman. ( Facebook: Trevor Ruthenberg - LNP for Longman )

Mr Ruthenberg's campaign website lists his six years of service in the ADF where he "trained as an aeronautical ground engineer".

"I've never worn the Australian Service Medal — I've only ever worn the medal that was awarded to me," Mr Ruthenberg said.

"I respect the uniform way too much to intentionally claim something I don't have.

"I'm not a military impostor. I did serve in the military.

"As soon as it was pointed out to me, I immediately stuck my hand up and said I've made a mistake and have apologised for that."

Campaigning in Longman with Labor candidate and former MP Susan Lamb, Queensland Premier Annastacia Palaszczuk said voters would judge Mr Ruthenberg's honesty.

"I think it's incredibly disappointing to see someone who essentially did not tell the truth," Ms Palaszczuk said.

"I think there would be many people at the Bribie RSL that would be shocked and dismayed that someone would have taken this action.

"He might say [it was an honest mistake] but that's going to be up to the voters of this electorate to work out what they think."

Queensland Health Minister Steven Miles went further.

"Now we learn that Trevor Ruthenberg lied that whole time about his military service," Mr Miles said.

Turnbull has spoken to 'Big Trev'

Prime Minister Malcolm Turnbull said he had spoken to Mr Ruthenberg this morning, saying it had been "an honest mistake".

"Big Trev is as honest and straight as he is big, and he is big," Mr Turnbull said.

Mr Turnbull told Radio 3AW earlier this morning that Mr Ruthenberg was "very embarrassed about it".

Sorry, this video has expired Turnbull says Longman candidate made 'honest mistake' in medal citation

"He feels bad about it — he has apologised profusely," Mr Turnbull said.

"He has made an innocent mistake."

Mr Ruthenberg admitted it could be a bad look heading into the by-election at the end of the month.

"That'll be up to the voters — they'll have to judge me on face value — unlike some of the candidates who haven't owned their mistakes, I have," Mr Ruthenberg said.

"That's up to the folks, I think they can see that I've done the best that I can do.

"I don't think it's a pretty big mistake, I think it's a mistake … it's sat there [on the Queensland website] as a sleeper since 2012."

Candidate convinced mistake won't affect polling

Mr Ruthenberg said he called Mr Turnbull to explain the situation.

"This is not the sort of front page news that the Prime Minister wants to see," he said.

"When you make a mistake you put your hand up and you own the mistake and that's what I'm doing now."

The former state MP said he was convinced it would not affect his chances at the upcoming by-election.

"Keep doing what I'm doing, I will out there talking to people we'll keep executing our plan," he said.

One Nation could be the beneficiary: expert

Australian National University political expert Andrew Hughes said the controversy could have an impact on the upcoming by-election.

"The question is whether it's going to carry through to polling day," he said.

"It may just be enough in some people's minds for them not to vote for the LNP.

"Let's not forget that prepoll voting is open and at last election [the] pre-poll voting rates were about 30 per cent of people."

Mr Hughes said One Nation could be the party that benefited most, with many voters already disenchanted by both major parties.

Tony Ferris from RSL Queensland said: "I don't know that it's an easy mistake, but at the end of the day, he's made a mistake and he's admitted to his mistake."

"He's served, he's entitled to a medal, whether that's the right medal or not, that's his problem."

Several hours after the story broke, Mr Ruthenberg's biography on the website was updated.

"In the hustle and bustle of sorting all your stuff out when you first get elected, and going through all the induction processes and things like that, it's an eye for detail that I've just missed," Mr Ruthenberg said.

"All I can do is apologise to those folks who earn the Australian Service Medal."