Ansett doesnot state which aircraft type a flight will be operated by in any of it's publicity including it's website (I booked a ticket on it 2 days ago).
In answer to your question, a friend of mine flew from MEL-SYD on a HKG bound flight and it was the 763, which I understand is infact in service currently. That was a few months ago. Certainly there is a daily 744 service from SYD, but I understand any flight that involves MEL is operated by the 763.
Hope that helps