Leighton Holdings announced Thursday the appointment of Hamish Tyrwhitt as chief executive officer and managing director. Tyrwhitt will immediately replace David Stewart, who resigned after barely eight months in the job following the 23-year reign of former chief executive Wal King.

Stephen Johns, meanwhile, has been elected to succeed David Mortimer as chairman. Leighton announced yesterday Mortimer has resigned from the board for personal reasons, without disclosing any other details. Mortimer, however, is believed to have resigned in protest at Stewart's ouster, which was confirmed Wednesday night.

Thursday’s announcement on Stewart’s abrupt departure stunned investors, sending shares in Australia's top contractor tumbling despite fresh assurances it would meet its 2012 profit forecast.

“As stated at the full year results on 15th August, Leighton has a very positive long-term outlook with $46.2 billion worth of work in hand, a strong competitive position and solid economic activity in the major markets in which it operates. The company re-confirms guidance for the 12 months to 30 June 2012 of an after tax profit of between $600-650 million,” Leighton said.

At 0145 GMT, its shares were down A$1.12, or 5.3 per cenr, at A$20.19, in an overall market up 0.8 per cent.