Nev, it may not be in CAO 48 but the following definition is lifted from the CASA 'Standard Exemption' :
2.8 Flight Deck Duty: Is the total time a flight crewmember is on duty on the flight deck in a flight duty period.
On single pilot or two crew operations it would be simple because it is the entire duration of the flight, including pre-flight and post-flight duties in the cockpit; however its real application is for augmented crew operations where the third pilot may or may not be on the flight deck for takeoff and landing, but is rotated through a bunk or rest seat.