He wrote the book, literally, on the cases that shaped the nation. Now the Sydney silk assisting the royal commission into trade unions is supplying some hearing room drama of his own.

Jeremy Stoljar SC has come under fire from ALP and union figures in recent days for his grilling of Opposition Leader Bill Shorten.

But the legal fraternity says the well-respected commercial barrister, who was hand-picked by Commissioner and former High Court judge Dyson Heydon to assist the commission, has no political axe to grind.

Hours after Mr Shorten squared off against Mr Stoljar on Wednesday, one of his backbenchers vented his spleen about the commission on Twitter.

It is part of a broader Labor attack on the legitimacy of the Abbott government-appointed commission. Senior figures have labelled it a "witch hunt" and an abuse of power aimed at destabilising the party and the union movement.

"Stoljar's poor performance at the Abbott TURC shows (it's) a $50m farce and waste of taxpayer money," tweeted the federal member for McEwen, Rob Mitchell. "It should end both their careers."

Earlier that day, Mr Shorten revealed under questioning by Mr Stoljar that he had not declared a donation of more than $40,000 used to pay his campaign manager.

In a sensational development on Thursday, Commissioner Heydon warned the Opposition Leader that "a lot of your answers are non-responsive". "What I'm concerned about is your credibility as a witness," he said.

He suggested Mr Shorten curb his "extraneous answers", saying that Mr Stoljar was entitled to pursue his plan for the examination.

Geoffrey Watson, SC, who assisted landmark inquiries by the NSW Independent Commission Against Corruption into senior Labor and Liberal figures, dismissed suggestions Mr Stoljar had been too aggressive in his questioning of Mr Shorten.

"An appearance at the royal commission isn't a trip to the day spa," Mr Watson said.

"I've watched quite a bit of his work on the live-streaming and I would disagree fundamentally that he's anything other than totally fair and appropriately firm."

Mr Stoljar is the author of The Australian Book of Great Trials: The cases that shaped a nation, which was launched in 2011 by Mr Heydon. Both hail from Eight Selborne chambers in Sydney's Phillip Street.

"That book is a corker," Mr Watson said. "He is a genuine intellectual who in his spare time has made a contribution to a broader understanding of the law."

Almost two decades earlier, Mr Stoljar penned a novel called My First Mistake, about a Nordic hero, which he dedicated "to the memory of my father", Samuel, a well-regarded legal academic at the Australian National University.

The University of Sydney alumnus was admitted to the bar in 1996 and took silk in 2008. He has plied his trade as a commercial barrister specialising in property and equity.

A senior barrister said Mr Stoljar began his legal career working at the Consumer Credit Legal Centre "battling for the consumer."

At the time the barrister was on the other side of the bar table, representing banks, and found Mr Stoljar a "formidable opponent and a very honest and competent opponent."

The barrister, who also knows Commissioner Heydon, said both would be "disgusted" if they knew people had criticised them as political pawns in their respective roles at the royal commission.

"Right-wing barristers aren't there for the preserve of the right-wing," he said.

"If a barrister did have right-wing leanings and a left-wing trade union briefed him...that barrister would be bound to accept that brief.

"That principle, known colloquially as the cab rank rule, is fundamental to the bar that anybody in the community, whatever their attributes or negative characteristics one might have is able to brief an available barrister for a trial or case."

Mr Stoljar had appeared for a wide range of commercial and equity law cases, but had not been briefed repeatedly by any section of politics or the community.

Another senior barrister, who has worked closely with Mr Stoljar for about 20 years and considers him a good friend, said he was someone with "genuine concern for vulnerable members of society."

But, like most barristers, he kept his political views private: "I wouldn't have a clue how he votes and I don't think anyone at the bar would know."

Mr Stoljar's pay packet at the Royal Commission came under scrutiny earlier this year, after tender documents revealed he would be paid $3.3 million for his work from last March to the end of this year.

Sustained criticism from Labor figures has focused largely on the commission rather than Mr Stoljar or Commissioner Heydon personally, although the latter has also been in the crosshairs of Labor figures in recent days.

Regarded as both a legal and a political conservative with a keen intellect and fierce independence, Commissioner Heydon was appointed to the High Court by the Howard Coalition government in 2003.

The Carr Labor government had appointed the University of Sydney and Oxford-educated QC to the Court of Appeal in 2000.

Shadow employment minister Brendan O'Connor said on Wednesday the commission was an "$80 million witch-hunt by the government (that) will not in any way impact on Bill Shorten."

The Australian Crime Commission and the Australian Federal Police were better placed, he said, to deal with any serious criminal allegations.

Mr Shorten is the third Labor leader to face a royal commission called by the Abbott government. Mr Stoljar cleared former prime minister Julia Gillard of any criminal wrong-doing after asking her about her dealings with union officials linked to a slush fund, but said some aspects of her work as a solicitor "appear questionable."

Kevin Rudd also previously appeared at a royal commission into the failed home insulation scheme.

Fairfax Media was unable to contact Mr Mitchell.

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