The department has been under scrutiny over its privatisation of the city's passenger train and tram network, which has taken place despite the Government claiming it did not have a privatisation agenda ahead of the 2018 state election.

Opposition transport spokesperson Tom Koutsantonis said the awarding of the contract to Keolis Downer had been subject to several leaks to MPs, including claims the $2.14 billion 12-year contract would ultimately cost more than it did to run the network publicly.

"None of this information came out in a press release, or the Government telling the public how it would spend its money," he said.

He said whistleblowers had told the Opposition that a private investigator had been hired by the Department of Infrastructure and Transport (DIT) to find the source of the leaks.

The Opposition, under Leader Peter Malinauskas, has consistently campaigned against privatisation.(ABC News)Neither confirmed or deniedABC Radio Adelaide approached Transport Minister Corey Wingard over the accusation but he instead deferred to DIT.

A DIT spokesperson said the department took "any potentially unlawful release of information seriously and referred such matters to relevant authorities".

Mr Koutsantonis said if something illegal had taken place, the police would have been called in, or the Office of Public Integrity.

"The reason they're hiring a private investigator isn't to investigate an illegality, it's to try and fire people," he said.

"I think when you're spending public money the public has a right to know how every dollar is spent ... it is in the public's interest."

Keolis Downer is a joint venture between the Downer Group and French transport company Keolis.

Labor has consistently campaigned against the privatisation and has pledged to reverse it if elected next year.