She made the claim for two years' pay just as the Abbott government's royal commission on union governance and corruption was set to begin on Wednesday with a formal statement by High Court Justice Dyson Heydon. The HSU is one of five unions to be investigated after the successful prosecution of former corrupt bosses Michael Williamson and Craig Thomson.

Ms Jackson, who took evidence of wrongdoing inside the HSU to police, lost her job as executive president of the HSU East branch in June 2012, when the union was placed in administration by Bill Shorten in his time as workplace relations minister.

She had been drawing a salary of $270,000 a year as president of HSU East but retained the title of national secretary of the union in an honorary, unpaid capacity. She has not worked in that role since 2012.

Her claim for two years' pay and sick leave entitlements surprised the union's new leadership, which will question whether an official can receive back pay for an unpaid role.

Ms Jackson said she had been given assurances when HSU East had been forced into administration that she would not be disadvantaged. ''This position is not honorary and that's what they [the acting national president Chris Brown and the HSU leadership] told the court in June 2012,'' she said.

The bad blood between Ms Jackson and the new guard appears certain to flow into the royal commission, with both preparing submissions. Ms Jackson said she will attend the first day's hearing.

The HSU is suing Ms Jackson in a civil case in the Federal Court over claims she broke the union's financial governance rules when she used union funds to hire law firm Toomey Pegg in late 2011.

The lawyers were engaged to prepare a response to proposed findings against Ms Jackson by a Fair Work Australia investigation. The HSU wants $35,000 from her.

The union has also alleged that she broke governance and reporting rules to appoint factional ally Rob Elliott to a $150,000 position when she was secretary of an HSU branch in Victoria. A hearing on that matter is set for May.

The Australian Workers Union, Construction Forestry Mining and Energy Union, Electrical Trades Union and Transport Workers Union, will be targeted by the commission. Prime Minister Tony Abbott said it needed to ''shine a big spotlight'' on the unions.