Bill Shorten isn't the first politician to be embarrassed by a late campaign declaration, and he won't be the last.

The difference is that most slip-ups aren't revealed under the glare of a high-profile royal commission into "corruption" when the subject is already under immense pressure.

Nor is he unique in soliciting, or accepting, donations from those who see it as in their commercial interests to contribute to political parties or individual candidates. Not by a long shot.

But he may be the first national leader to have his integrity on electoral matters challenged by an aggressive senior counsel during an inquiry instigated by his political opponents.

The double damage for the Labor leader is that the late disclosure concerned a payment that was misrepresented – and both were revealed on Mr Shorten's first day in the hot seat.

To assess the impact, there was no better barometer than Mr Shorten's body language.

Before the lunch break, the Labor leader projected a calm confidence that he could answer any questions the commission might care to put about his conduct as a union official before he entered Parliament.

In several exchanges, he either held his own or had the better of senior counsel assisting the inquiry, Jeremy Stoljar, as he explained the context of the time and his motivations.

No response was more emphatic than when Mr Stoljar suggested that he had used his position to gain an advantage for himself in the form of a full-time campaign worker, courtesy of an employer who was negotiating an agreement with his union (but not with him).

"Absolutely not," he declared.

Immediately after the lunch break, Mr Shorten was like a fighter who had taken a heavy blow to the solar plexus; the stuffing knocked out of him by the revelation that his candidate's record for the 2007 election had only been finally corrected on the eve of his appearance. He seemed defeated.

No wonder the quietly spoken commissioner invited him to take the break that gave him the chance to regroup.

On a one-to-10 scale of electoral misdemeanours, what was revealed was at the lower end: no smoking gun of corruption, but an error compounded by a stuff-up. A bad look, but let he (or she) in the Coalition who has not submitted an amended return cast the first stone.

Mr Shorten denied being responsible for misrepresenting the employment of the campaign worker and was unable to explain why the employment contract described him as a research officer for the company.

He pleaded ignorance for the failure to disclose, too, insisting that when he discovered the incomplete return he rectified it.

He also pointed out that it was common for MPs to assert they had nothing to declare in their electoral returns because funding records were collated by parties centrally and then disclosed.

Tony Abbott, Mr Shorten told the commission, had filed a "nil" return for the last three elections.

The more substantial question is whether Mr Shorten acted in the best interests of his members when he led the Australian Workers Union and, while it is premature to draw conclusions, there is an almost surreal edge to this inquisition.

Here is the party that wanted to abolish the no-disadvantage test for enterprise agreements taking Mr Shorten to task for allegedly failing to uphold it in one instance.

For Mr Shorten, another day in the hot seat beckons.

One question he can be confident that will not be asked is whether there is a better, fairer and more cost-effective way to expose bad practice, secure transparency and hold our politicians to account.

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