The geodetic parameters between the two are not the same, which is likely the main reason why. This will probably also be an issue for any form of GNSS as it will be still different to the WGS84 geodetic parameters.
Reading the GDA1994 Technical Manual, accuracy between the two is in the order of 1m (even if you were to do a 7 Parameter Transform).
Personally, I think you might be out of luck here without some more complicated work.