Senator Pratt, who conceded she would lose her seat, said Mr Bullock's installation at the top of Labor's ticket of WA Senate candidates highlighted the need for deeper democratisation of the party's structures.

Senator Pratt was forced to take the second position on Labor's Senate ticket because of a factional deal between Mr Bullock's union, the Right-aligned Shop, Distributive and Allied Employees Union (SDA), and the Left-aligned United Voice union.

The power of trade unions within Labor – especially the SDA – had undermined public faith in the party and its ability to win elections, Senator Pratt said.

"It is a blow to progressive voters that I would be replaced in the Senate by someone who I have known for many years to be deeply homophobic, to be anti-choice and, has recently emerged, disloyal to the very party he bas been elected to represent," she said.

"Far from my views on these issues being fringe as he has claimed, it is Joe Bullock and other members of the SDA leadership who are on the fringe of mainstream views."

In a speech that went public the day before the WA Senate rerun, Mr Bullock described Labor party members as "mad" and said he did not always vote for the party.

The national secretary of the Shop Distributive and Allied Employees Association Joe de Bruyn on Thursday defended the preselection process in WA as democratic, and said that there were good arguments for keeping the existing system rather than giving a greater say to rank-and-file Labor members.

“Half of the people there are rank-and-file members and when the preselection was conducted Joe Bullock won a two-thirds majority,” he told ABC radio.  .

Mr de Bruyn also defended Mr Bullock following comments the incoming senator made about same-sex marriage and Ms Pratt.

“I’ve known him for 35 years and he is not homophobic,” he said.

“I know that he has frequently represented members of the union who are homosexual and he is deeply committed and hard working and goes out of his way to assist those people like he does any other members.”

Labor Senate leader Penny Wong joined calls for internal reform, arguing in a seminar on Tuesday night that rank-and-file Labor members should have a say in Senate preselections.

"I think it's going to be hard to explain to the Australian public why members of our party have a direct role in the election of the parliamentary leader – the woman or man who would be prime minister – but not in the preselection of senators and other candidates for public office," Ms Wong said.

Labor frontbenchers Chris Bowen and Kim Carr have previously supported giving Labor Party members a vote in Senate preselections.

Fairfax Media revealed on Wednesday that Opposition Leader Bill Shorten will call for all preselections to move to a 100 per cent rank and file model in the longer term and will urge the party to broaden the talent pool from which it preselects senators.

The pro-democratisation push is facing resistance from union leaders including Transport Workers Union national secretary and NSW Right powerbroker Tony Sheldon.

On Wednesday, Mr Sheldon, who is also national vice-president of the Labor Party, said party reform was an "insiders' debate" and that giving members more power over preselections would "not change the world".

“I believe the present system – particularly in NSW – is appropriate and proper,” Mr Sheldon told the National Press Club.

While he said he disagreed with Mr Bullock's views on gay marriage, Mr Sheldon said he should remain in his Senate seat.

"We are a broad church and should continue to be a broad church regardless of how much I disagree with Joe Bullock," he said.

"As someone who has said some rather provocative things about some of our own leaders in the Labor Party … I wouldn’t hang somebody for words that might have been better left unsaid."

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