Ferguson's exit was followed by the resignation of Chairman Mark Chellew as the company faces a public inquiry initiated by the New South Wales corruption watchdog.
The regulator is investigating allegations that certain Downer employees obtained personal benefits by favouring subcontractors for contracts awarded by the transport department.
Ashcroft has previously served as the finance head of local peer Healius and is currently the chief operating officer at G8 Education, Downer said.
Downer has also reported accounting irregularities in its utilities business.
(Reporting by Rishav Chatterjee in Bengaluru; Editing by Sriraj Kalluvila)
