Thomson, HSU investigation cost $4 million

The Fair Work Commission has spent more than $4 million on investigation and litigation into Craig Thomson and the Health Services Union.

The Fair Work Commission has spent more than $4 million on a probe into Craig Thomson and the HSU. (AAP)

The cost to taxpayers of the Fair Work Commission investigation into former Labor MP Craig Thomson and the Health Services Union has exceeded $4 million.

Commission general manager Bernadette O'Neill told a Senate estimates hearing in Canberra on Thursday $4,136,000 has been spent.

Mr Thomson's case, regarding the misuse of union funds, will return to the Federal Court on November 9.