The Royal Commission into Trade Union Governance and Corruption will begin its investigation of the activities of the Construction, Forestry, Mining and Energy Union (CFMEU) today.

The inquiry, led by former High Court judge Dyson Heydon, will look at allegations of bribery, secret commissions and improper fundraising levelled at five of Australia’s most powerful unions.

The commission’s CFMEU probe will run for three weeks, during which time it will hear from almost 40 witnesses.

ACT branch secretary Dean Hall will be one of the first to give evidence, but more than half of the union's branch staff including the assistant secretary and several organisers are expected to appear.

Master Builders Association representative John Nickolic will also appear before the commission.

The Royal Commission has held hearings nationwide in the last 18 months, and has been labelled a “witch hunt” by unions and the Labor Party.

The latest round of hearings comes just day after the high profile questioning of Federal Opposition Leader Bill Shorten.

Mr Shorten was grilled over payments the Australian Workers Union received from various companies during his time as its state and national secretary.