Labor MPs are privately acknowledging the Opposition Leader has “lost some bark” during his appearance at the royal commission into union corruption, but they maintain the damage is not serious.

Bill Shorten has spent two days giving evidence at the commission hearing in Sydney, about his time as the Victorian and national secretary of the Australian Workers Union (AWU) before he entered politics.

He may also be recalled to give further evidence before the commission’s final report is due at the end of the year.

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During the inquiry, Mr Shorten admitted that the Labor Party failed to declare a business donation of a senior staff member to his 2007 parliamentary election campaign.

The party only made the disclosure this week, eight years on.

He has also been accused of a potential conflict of interest, because the union accepted $500,000 in payments from a glassworks company for ‘fees’ and ‘training’ while negotiating pay deals for workers.

The commissioner, Dyson Heydon QC, further criticised Mr Shorten for providing “long and extraneous” answers, and over his credibility as a witness.

Government frontbencher Eric Abetz said he thinks most people would be “horrified” by some of the evidence exposed through the royal commission.

But Mr Shorten has strongly rejected the accusation he has acted improperly.

At the end of the marathon hearings he fronted reporters to defend his reputation and declare he had “no conflict of interest whatsoever”.

“There was no evidence demonstrated of any conflict,” Mr Shorten said.

“The truth of the matter is that every day I was a union rep I was standing up for our members.”

Sources within the Labor Party have told the ABC that Mr Shorten has “lost some bark” through the process, and that it is “unhelpful” for the alternative prime minister to be hauled before the inquiry and the cameras.

But they do not think the damage to his credibility is serious and maintain there was “no smoking gun” or “hanging offence” in the commission hearing.

Labor frontbencher Brendan O’Connor has been a key public defender of the Opposition Leader while the royal commission hearings have been underway.

“He [Mr Shorten] acquitted himself well in the last two days, he was credible, cooperative and candid,” Mr O’Connor told 7.30.

Mr O’Connor was questioned about the impression the coverage of the commission will have on voters.

“Perhaps there might be people who form an unfavourable impression,” he said.

But he said voters would be more concerned about the Government’s decision to pursue Labor through the royal commission process.

“I think increasingly people are starting to question the fact that there’s been three Labor leaders called to Tony Abbott’s royal commissions,” Mr O’Connor said.

Labor backbencher Nick Champion gave his assessment of the impact of the royal commission by saying: “My prediction is [it] will be like a … bit of rough wind — it’s going to blow in one side and out the other. With very little effect.”

The strongest internal condemnation of Mr Shorten so far has come from former Labor national secretary Bob Hogg, who wrote an open letter calling for the Opposition Leader to quit, and posted it on his Facebook page on Wednesday night.

Mr Hogg held the senior ALP position between 1988 and 1993.

Labor senator Sam Dastyari said Mr Hogg had to be careful of “throwing stones” because he had also failed to declare political donations in the past.

“Look, there’s nothing as ‘ex’ as an ‘ex’,” Senator Dastyari said.

“This morning I had to scratch my head and try and remember the last leader that Bob Hogg didn’t call for the resignation of.”

Senator Dastyari claims the royal commission is eroding the political process, but he remains confident Mr Shorten will ride out the intense scrutiny this week.

“What we’ve seen here is the Americanisation of Australian politics, where incumbents use these kinds of processes, ‘show trials’, to smear political opponents,” Senator Dastyari told 7.30.

“Let’s not pussy foot around this issue. Bill Shorten is not only going to lead Labor to the next election. He’s going to win it for Labor.”

Mr O’Connor said rather than suggesting Mr Shorten had a conflict of interest, the same accusation should be levelled at the Government.

“I do believe it is a conflict of interest for the Prime Minister to spend $80 million of taxpayers’ money to go after the political opponents of the Government,” he said.