West Australian Labour Senator Joe Bullock on Tuesday quit politics over his party's position on same-sex marriage.
Bullock has been a right-faction power broker in the Labour Party and entered the Senate in 2014.
He said that he had long been against same-sex marriage and could not remain a Labour member of parliament, given that the party will remove its members' ability to vote with their conscience on the issue in 2019, Australian ABC reported.
"Instinctively I know if your job requires you to do which you believe to be wrong, there's only one course of action: resign," he said.
"As a member of the party, I'm free to disagree with the party policy, to lobby for change, and to encourage people to join the party with a view to achieving that end."
However, he said, as a member of the Australian parliamentary Labour Party, he had different obligations.
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"As a Labour Senator, it's my job to tell voters that it doesn't matter that the Labour will outlaw the conscience vote on homosexual marriage, and to recommend a vote for Labour without reservation.
"That's the job description of a Labour Senator. It's a job which I can't do," he said.
Before entering politics he commanded significant power within the right faction of the West Australian branch of the Labour Party through various leadership roles within the right faction-aligned Shop Distributive and Allied Employees Association.
His patronage has got other West Australian Labour figures into the parliament and in 2014 he stepped out of backroom roles and into office himself.
But Senator Bullock, a man with conservative views on abortion and same-sex marriage, soon found Canberra a lonely place.
As recently as last week, he found himself at odds with his party after he called for the safe schools anti-bullying programme to be "immediately stopped".
He said that it was a terrible programme "so narrowly focused on homosexual issues that it doesn't provide the sort of balance one would hope".
Labour's leader in the Senate, Penny Wong, criticised him over the comments, saying: "I don't agree with Joe and the Labour Party doesn't agree with Joe."
Opposition Leader Bill Shorten released a statement thanking Senator Bullock "for his years of service to the Labour Party and decades representing some of Australia's lowest paid workers".
"I don't agree with his views on a number of issues -- including marriage equality -- but I respect his right to hold those opinions," Shorten said.