'Be aggressive': corruption buster on Big Build inquiry

A long-awaited inquiry into alleged corruption on taxpayer-funded construction projects is hours away from facing its first major test of public confidence.

So says Geoffrey Watson SC, the corruption-busting barrister who estimated $15 billion had been rorted on Victoria's "Big Build" infrastructure program.

His investigations helped set the stage for a royal commission into the integrity of major state projects linked to the CFMEU's Victorian branch.

Ahead of the inquiry's opening hearing on Wednesday, Mr Watson said it was crucial commissioner Christopher Kourakis hit the ground running.

"I wouldn't propose giving advice ... but you've got to be very robust (and) quite aggressive," Mr Watson told AAP.

At a cost of $50 million, he said it was crucial the public had faith the commission would not pull any punches.

"You've got to win your confidence early and keep it," the Centre for Public Integrity director said.

The opening hearing would be closely watched for signs of the commissioner's intent, including by people who might hold relevant information but have so far stayed silent, Mr Watson said.

He said the commissioner would make assurances that information may be treated confidentially and security could be arranged.

Premier Ben Carroll in July promised to set up the royal commission as his first act in government, after toppling his predecessor Jacinta Allan.

Ms Allan had for months rejected widespread calls to launch a probe into allegations of dodgy dealings within the state's $100 billion infrastructure program.

It followed years of reporting on alleged wrongdoing on Big Build sites, linked to the Construction, Forestry and Maritime Employees Union's Victorian branch.

An explosive report by Mr Watson titled Rotting from the Top estimated alleged corrupt conduct by the CFMEU had effectively fleeced Victorian taxpayers of $15 billion.

Mr Watson said he felt "relieved" to finally see the accountability wheels turning.