I'd cross floor on gay marriage: Lib MP

Openly-gay Liberal MP Trent Zimmerman says he would cross the floor in favour of same-sex marriage.

"If a substantive bill came before the parliament, then yes I would if the party instructed me otherwise," he told ABC radio on Wednesday.

Mr Zimmerman said he was concerned about a plebiscite on gay marriage and his personal preference, had he been in the party room earlier, would have been for a free vote and for parliament to decide the issue.