My reading of the words of the rules is that it is a "Circle Trip" fare which means that you cant go through the same city twice. On this interpretation your HKG-BNE-SYD bit is OK because you are not backtracking, but your Marseille side trip would be an additional charge.

On the other hand, the route map which accompanies the words doesn't show BNE-SYD as a valid element of the routing, so your TA may be right.

On the third hand ( ) my TA has managed to get BA to be pretty flexible on this type of ticket in the past. For example, the rules seem to say that the only stopover allowed between UK and Aus is Singapore, but she got BA in HKG to ticket:

HKG-LHR-MNL-SYD-HKG

in the days when BA flew to MNL via HKG. The LHR-MNL was a single coupon but I have a feeling I just got off at HKG and got to MNL on another ticket later, which was a bit naughty.

If the routing fits what you need then these tickets are pretty good value in business class particularly.