A defiant Bill Shorten has dared Tony Abbott to "bring it on" and fight Labor head-to-head over worker's rights, declaring he "relishes" the chance to front the trade union royal commission on Wednesday and "talk about what I believe to be every Australian's right – a good, safe job with proper pay and conditions".

Ahead of the biggest test of the Opposition Leader's political career, Mr Shorten vowed Labor "will never give up the fight for decent pay and conditions at work, never".

"If Tony Abbott and the Liberals want a contest on who will look after working people in this country, bring it on. Labor will always win. I'll put my record against Tony Abbott's anytime – today, tomorrow, any day of the week."

Prime Minister Tony Abbott, in turn, sharpened his political attack on Tuesday, challenging Mr Shorten to explain the details of deals struck while he was head of the Australian Workers Union.

"In the end, what we want to come out of this is honest unions that do the right thing by their members and, plainly, it seems that there have been a lot of ghosts on the rolls of some of the unions, there have been some deals that have been done to help the unions but to dud the workers," he said.

The royal commission is expected to quiz Mr Shorten on a range of issues including more than $1 million in payments from employers to the AWU's Victorian branch between 2004 and 2007 when he ran the union.

Those payments included a $300,000 payment from builders Thiess John Holland to the union after a deal was struck that saved the company as much as $100 million on the $2.5 billion EastLink project. Former Connect East chairman Tony Shepherd, which subcontracted to Thiess, has strongly defended that deal.

Mr Shorten was called to appear before the commission after Fairfax Media revealed the AWU had received $38,228 from Winslow Constructors in 2005 to pay the union dues of 105 of its employees.

Current state secretary Ben Davis has told the commission those payments "profoundly" weakened the AWU's industrial position. The AWU's fund raising activities are also likely to be examined in what may not be Mr Shorten's only appearance before the commission.

Labor treasury spokesman Chris Bowen and former leadership rival Anthony Albanese both backed Mr Shorten on Tuesday, describing his leadership as "outstanding" and predicting he would "set the record straight" about his time in the union movement.

Opposition workplace spokesman Brendan O'Connor said the commission was "an $80 million witch hunt. The Coalition has spent five times more on this than on the national awareness campaign to reduce domestic violence, which has a budget of $16 million".

"This is a massive waste of resources and an attack on the government's political opponents. The last three Labor leaders Tony Abbott has faced have all appeared before royal commissions now," he said.

Labor elder and former party leader Simon Crean said the Opposition Leader should be proud of this time in the union movement and should use the appearance "as an opportunity to demonstrate how his approach didn't just benefit members, but was to the benefit of the economy as it addressed the need for competitiveness and productivity improvements".

Potential damage

Privately, Labor MPs contacted by Fairfax Media were supportive of their leader but admitted Mr Shorten's appearance could cause significant damage to his standing with voters and predicted it could form the basis of political attack ads in the next election campaign.

One MP reiterated concerns Mr Shorten could suffer "death by a thousand cuts, though there doesn't seem to be proof that he did the wrong thing" by appearing before the commission.

" There seems to be no sense that there is a smoking gun out there. We will see what the evidence is but I think Bill will come out strongly," the MP said.

Another federal Labor MP described Mr Shorten's royal commission appearance as "unhelpful", but said the party was more focused on its upcoming national conference.

"The real challenge for Bill is not going to be a performance at the royal commission. It will be presenting a positive agenda at ALP conference," the MP said.

"That's when all eyes will be on Bill."

Several MPs seized on Monday's Fairfax-Ipsos poll, which found Labor still leads the Coalition 53-47 in the two-party preferred vote and Mr Shorten enjoyed a small lead as preferred prime minister.

But the poll also found Mr Shorten's approval rating has slipped below Mr Abbott's, his trustworthiness rating with voters took a 5-point hit and a massive 46 per cent of people believed the Opposition Leader was easily influenced by minority groups.

Mr Shorten is not the first political leader to front a royal commission. Bob Hawke testified to the Hope royal commission in 1983, John Howard fronted the Australian Wheat Board inquiry in 2006 and former NSW premier Neville Wran faced the Street royal commission in 1983.

Mr Shorten's leadership position is protected by ALP rules introduced in 2013 that require 60 per cent of the caucus to sign a petition calling for a spill.

A challenger would then need to win more than 50 per cent of the caucus vote.

It would take a matter of serious impropriety to arise during the commission hearings for a situation to arise where he would be expected to fall on his sword, like the former NSW Labor leader John Robertson.

Mr Shorten will be represented by law firm Arnold Bloch Leibler and Ray Finkelstein QC, who are providing their services on a pro bono basis.

Visit The Pulse from 9.30am for full live coverage of Bill Shorten's Wednesday appearance before the royal commission.

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