The Greens and gay activists have welcomed a vow by a Tasmanian Liberal candidate to take up the fight for gay marriage.

The Liberals' Denison candidate Cameron Simpkins has not ruled out crossing the floor of parliament in support of gay marriage.

Just days ago Federal Opposition leader Tony Abbott reaffirmed his opposition on commercial radio.

"I guess I'm old fashioned in the sense that I think marriage is about a man and a woman," he said.

Mr Simpkins says if it came to it he would be prepared to cross the floor.

"If it was put to a conscience vote then I would align myself with that side yes."

The former army major acknowledges it would be difficult to win support for his views within the party but thinks it is time the party re-opened the discussion on same-sex marriage.

"I'd be the new boy on the block inside a party room," Mr Simpkins said.

"If I am lucky enough to be elected and the discussions were held then I would state the case and this is not about gay marriage this is about prejudice and I'm against prejudice of any kind."

"I think that conversation needs to be had and it needs to be had in this next term of government, we need to to have those discussions."

Mr Simpkins told a forum on Tuesday night he would cross the floor on the issue but yesterday told reporters it would depend how the question was framed.

Greens Senator Christine Milne says Mr Simpkins' view is a welcome change.

"We have had the extremely conservative side of the Liberal Party dominating Liberal politics in Tasmania for way too long, and it is refreshing to see a small 'l' Liberal stand up and support marriage equality."

Tasmanian gay activist Rodney Croome has also praised Mr Simpkins.

Mr Croome hopes other candidates will follow suit.

"We know for a fact that there are many candidates for both major parties who personally support same-sex marriage," he said,"but they're not willing to say that because they're afraid of the repercussions, particularly within their own party."

Tasmania's most senior Liberal has also reaffirmed his opposition to gay marriage.

Senator Eric Abetz has released a statement saying he believes the traditional view that marriage is a heterosexual union.