Almost $28m spent investigating unions

Almost $28 million has already been spent by the royal commission investigating union corruption.

The Royal Commission into Trade Union Governance and Corruption on Wednesday released an operational update, including details of the money so far spent probing the unions.

Operating expenditure, as at 30 June 2015, was approximately $28 million.

That includes funding for the Office of the Royal Commission, police taskforces in NSW, Victoria and Queensland, and the financial assistance to witnesses provided by the Attorney-General's Department.

So far, four arrests have been made as a result of the commission.

The $28 million also includes the funding of lawyers, as well as the commissioner, Dyson Heydon.

It does not include money spent by the Australian Federal Police.

The commission's total operating budget is approximately $61 million over two years.