Four senior executives from Downer EDI’s Spotless and rival contractor Ventia Services have been named in a case over alleged price-fixing brought by the competition watchdog relating to multibillion-dollar contracts with the Defence Department.

Shares of both companies tumbled on Thursday. Ventia shares fell 23 per cent to $3.31 – its lowest price since February – while Downer’s stock slid 6 per cent to $5.37.