Realised I had a few screenshots of the destination maps saved locally from the last time I went to plan the free flight. Will include them here in case the visuals make it easier for some people (I know I certainly liked looking at the map, not just the destination list).
Here's from Sydney:
Here's from Melbourne:
Here's from Brisbane:
Here's from Perth:
Obviously the maps are a tiny bit out of date (e.g. VA no longer flies to Ayers Rock), but hopefully they help.

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EDIT: Just spent an hour on hold waiting to book a Darwin flight, only for the operator to say they couldn't see one on the date I wanted (they said they could only see an indirect flight, even though a non-stop was on sale and had T9 availability). However, the operator hadn't even asked for the date of the returning flight, so it seemed they also weren't searching for the correct fare in the first place. They couldn't put me through to anyone else so I've asked them to have somebody else call me back, sigh.
ANOTHER EDIT: Didn't want to wait so called back and spoke to somebody else who was much more helpful. They had a similar problem with not seeing the flight I wanted (which was showing T9 in ExpertFlyer), but after checking on the VA site that a flight did indeed exist, they found a workaround: first, they booked an indirect flight (I wanted BNE-DRW, so they had to book BNE-MEL-DRW), then they got hold of their VA liason to switch it to BNE-DRW direct before issuing the ticket. End result, I have my free return flight booked as BNE-DRW-BNE, as planned.
Sharing in case anyone else runs into a similar issue with "computer says no", particularly with only a few days left to get any final free flights booked!