The federal opposition has called for a police investigation into Prime Minister Julia Gillard's role in establishing a union association her former boyfriend used to steal hundreds of thousands of dollars.

Shadow attorney-general George Brandis claimed yesterday that Ms Gillard may have broken the law by vouching for the bona fides of an association she later confirmed was a union election slush fund.

He urged Slater & Gordon to release all documents relating to the establishment of the Australian Workers Union Workplace Reform Association, which Ms Gillard - then a partner with the firm - had helped form.

An exclusive report in The Saturday Age revealed that the association was registered in 1992 only after Ms Gillard wrote to West Australian authorities confirming the association was a non-profit body dedicated to workplace safety and reform.

In September 1995, Ms Gillard confirmed to senior partners at Slater & Gordon that the association was in fact a "slush fund" set up to raise funds.

WA police later confirmed Ms Gillard's former boyfriend, senior AWU official Bruce Wilson, had stolen more than $400,000 from the association.

Senator Brandis said that if Ms Gillard knew that the association was designed to be a slush fund, it was not possible for her to have honestly described it to WA authorities as existing to promote workplace welfare.

He said she may have breached the WA Associations Incorporation Act, which made it an offence to make false or misleading statements.

Lawyers representing Ralph Blewitt, the former WA secretary of the AWU and associate of Wilson who registered the association, have been trying for more than a month to gain access to documents held by the firm.

Gillard's Legal Role 'Demands Inquiry'