Pat O'Neill defeated former ALP council candidate Philip Anthony in the Labor preselection for the federal seat of Brisbane on Monday and, if elected, would be the first openly gay member in Australia's lower house.

Mr O'Neill said he was well aware of the symbolism that would come with his possible election.

"I didn't come out until quite late in life, well I'm only 34 at the moment so not that late in life," he said.

"…I see it as a way to show, particularly for younger people, that you can be professionally successful, you can be happy and you can achieve whatever it is you want to achieve, regardless of your sexuality.

"Even though I'm not a one-issue person, I do think it's an important part of who I am.

"But it's only one part of who I am and I do think that being the first, or some kind of role model, would be a really healthy thing for people who may be in a situation where they're thinking they have to make a choice between their job and their personal happiness."

But first, Mr O'Neill would have to wrest the inner-city seat from the LNP's Teresa Gambaro, who holds it with a 4.28 per cent margin.

Labor has been on the outer in Brisbane since Ms Gambaro defeated Arch Bevis at the 2010 election, which was held in the aftermath of Kevin Rudd's removal as prime minister.

"I think we've got a good chance of winning back Brisbane," Mr O'Neill said. "We're going to be doing everything we can to engage on the issues."

Mr O'Neill has been a member of the Australian Army since he left school about 17 years ago and has reached the rank of major.

He had two tours in Iraq, working with a security detachment in Baghdad and Overwatch Command in southern Iraq, and also commanded Australian Army forces assisting in the clean-up following the 2013 Bundaberg floods.

Mr O'Neill said although he would have to resign from the army once the election writs were issued, it was an easy decision to make.

"I grew up in central Queensland, where my mum was a teacher at an Aboriginal school at Woorabinda for a number of years," he said.

"Growing up in central Queensland and being in charge of soldiers from different backgrounds, I realised I'd been extremely lucky to have the opportunities I had, the teachers, the education, the family.

"I know if I was born into different circumstances, I wouldn't have the chance to run in politics."

Mr O'Neill said he believed those opportunities should be afforded to everybody.

"Politics is the way that I see you can achieve that," he said. "Also, from a bit of a selfish perspective, the thing I enjoy most is working with people, working with community groups."

Coming out as a gay man in the Australian military in 2008 was not, Mr O'Neill said, a negative experience.

"Since I came out, the military has been exceptionally accepting and an exceptionally positive environment to be in and I cannot speak highly enough of my experience in the military since I came out," he said.

"Any issues I might have had before were probably my own internalised issues as opposed with issues with the military, but since coming out I couldn't speak more highly of the military as an organisation."

Marriage equality was a subject Mr O'Neill was particularly passionate about and he backed the push for Labor MPs to be denied conscience vote on the issue and instead be bound to vote in favour.

"As a party and as a country, wherever we can we should stand for equality, whether it's things like addressing the gender pay gap, gender discrimination or marriage equality," he said.

"It's not so much the issue of marriage equality, it's the issue of fundamental rights and everyone, no matter who they are, as an Australian citizen should be afforded the same rights as every other citizen.

"So I don't see any issue of equality as being a conscience issue. I see those issues of equality, no matter what they are, as being a fundamental bedrock of the Labor Party and a fundamental bedrock of Australia."

Mr O'Neill said he was not concerned that stance could make it more difficult to pressure the Liberal Party to allow its own conscience vote.

"They've shown through previous actions that they don't support these things," he said.

"They're not supporting a vote, so I don't think saying we should have similar standards or have the same expectation of Labor members as we do for the Coalition washes.

"I have much higher expectations for the Labor Party than I do for the Coalition."

It is understood Ms Gambaro, who has been a vocal critic of Prime Minister Tony Abbott, faces a preselection challenge from National Retailers Association chief executive Trevor Evans.

But an LNP spokesman said the party was still engaged in its vetting process.

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