Macmahon CEO Ross Carroll has resigned from the company effective immediately.
No reason was given for Carroll's abrupt resignation.
Macmahon chairman Jim Walker will act as executive chairman until a replacement CEO is found.
There have been additional changes at the board level, with deputy chairman Barry Cusack also resigning from the contractor.
Macmahon added: "To reduce costs the company does not intend to seek a replacement for Mr. Cusack's position at this time."
Commenting on the shake-up at the top, Walker stated that "the board is committed to implementing cost saving measures across the business as the company navigates the current suppressed market for mining services".