West Australian Labor senator Joe Bullock has announced he is quitting politics over his party's position on same-sex marriage.

Senator Bullock has been a right-faction powerbroker in the WA Labor Party and entered the Senate in 2014.

He made the announcement in the Senate on Tuesday night, saying he had long been against same-sex marriage and could not remain a Labor parliamentarian, given that the party will remove its members' ability to vote with their conscience on the issue in 2019.

"Instinctively I know if your job requires you to do [that] 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 parliamentary Labor Party, he had different obligations.

"As a Labor Senator, it's my job to tell voters that it doesn't matter that Labor will outlaw the conscience vote on homosexual marriage, and to recommend a vote for Labor without reservation.

"That's the job description of a Labor Senator. It's a job which I can't do."

Former WA senator Louise Pratt, pushed down the ALP ticket by Senator Bullock at the last election and not re-elected, will seek preselection to replace him.

Senator at odds with party over Safe Schools

Senator Bullock has loomed large in WA politics for many years.

Before entering politics he commanded significant power within the right faction of the WA branch of the Labor Party through various leadership roles within the right faction-aligned Shop Distributive and Allied Employees Association.

His patronage has got other WA Labor figures into 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 program to be "immediately stopped".

He told News Corp it was a terrible program "so narrowly focused on homosexual issues that it doesn't provide the sort of balance one would hope".

Labor's leader in the Senate, Penny Wong, criticised him over the comments, saying: "I don't agree with Joe and the Labor Party doesn't agree with Joe."

In 2014, he was forced to apologise "unreservedly" for attacking former Labor Senator Louise Pratt at a Christian function in Perth.

Opposition Leader Bill Shorten released a statement thanking Senator Bullock "for his years of service to the Labor 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," Mr Shorten said.

Senator Bullock is the fourth WA MP to announce they are quitting politics, following the retirements of Gary Gray, Melissa Parke and Alannah MacTiernan.

Mr Gray said Senator Bullock made a good contribution and his party should be more accommodating of MPs within its ranks who hold differing opinions.