Former prime minister Julia Gillard has maintained her innocence during a day of questioning about a union slush fund, but says there are things she would do differently if she had her time again.

“None of us get to go in a time machine and go backwards,” Ms Gillard told the Royal Commission into Trade Union Governance and Corruption on Wednesday.

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“Obviously if one got to do the whole thing again you would do things differently, given what I know now, but I did not know at the time.”

Ms Gillard was responding to questions from counsel assisting the commission, Jeremy Stoljar SC, about off-the-books work she did as a young lawyer for her former boyfriend, Australian Workers’ Union official Bruce Wilson.

Ms Gillard advised Mr Wilson and his union bagman, Ralph Blewitt, on setting up the AWU Workplace Reform Association (WRA) when she worked as a solicitor at Slater and Gordon in the early 1990s.

Mr Wilson used the incorporated entity to run a slush fund.

Ms Gillard’s failure to open a file for the work on the office system and rumours money from the fund paid for her renovations have dogged her career across two decades.

Over more than three hours of at times combative interrogation, Ms Gillard said she never suspected Mr Wilson and Mr Blewitt of wrongdoing and was only acting on their instructions.

At the time, she said, she believed the WRA was a fund to help Mr Wilson fight union elections in Western Australia.

Ms Gillard has said she ended the relationship when she became aware she had been deceived.

The former PM also defended not opening a file for the work she did, saying free or discounted services for union officials were common at the firm.

“I did more substantial jobs than this for free for trade unions during my days at Slater and Gordon,” she said

The commission has heard Mr Wilson and Mr Blewitt used the fund to channel undisclosed payments from a construction firm and that money from the fund was used to buy a house in Fitzroy, Melbourne, for Mr Wilson.

Witnesses also testified fund money paid for renovations at Ms Gillard’s home in the Melbourne suburb of Abbotsford in 1994 – claims Ms Gillard has denied previously and rejected again on Wednesday.

“All payments made for renovations on my property were from my own money,” she said.

In a 1995 interview with Slater and Gordon’s senior partners, conducted during an internal investigation as rumours swirled about the renovations, Ms Gillard said she “can’t categorically rule out that something at my house didn’t get paid for by the association or something at my house didn’t get paid for by the union”.

But on Wednesday she said she had taken further steps since 1995, including examining her own financial records, to satisfy herself that she did pay for the renovations.

Ms Gillard challenged the veracity of evidence by other witnesses about her renovations, including calling a statement by Mr Blewitt – that he went to Ms Gillard’s house to hand over cash to builders while Mr Wilson and Ms Gillard were home – “inherently improbable”.

“I worked full-time, I worked hard as a lawyer at Slater and Gordon,” she said.

“Mr Wilson worked hard as a union official. There would not have been times that we were sitting at my home while tradespeople were working.”

She rejected a suggestion from Mr Stoljar that she had told a builder who worked on her home, Athol James, that Mr Wilson was paying for the renovations.

“That’s completely untrue,” she said.