Thanks for the above. As a followup it worked out fine going through Sydney. Doing immigration was a bit wonky. If one had a chip passport one could go to a kiosk first, then go through immigration or go to a "manual" immigration line. The wonky bit was that there were kiosks along the way to immigration rather than at immigration. I finally asked and was told that the doing the kiosk first would be faster. Clearing immigration and customs was easy enough as the line moved along.

As I was flying on Virgin domestically I was able to checkin and drop off my bag at a counter just after exiting immigration and customs. I think the was also the same option for Quantus. Just outside was the shuttle over to the domestic terminal. I noticed some people had bags on board the shuttle. The shuttle over to the domestic terminal was easy enough. I arrived with plenty of time to walk around and stretch my legs. I think I had 90 minutes before my onward flight. So all-in-all it worked out fine.

On the return I did the opposite. Except I overnighted in Sydney before my international return flight. I stayed at the ibis Sydney Airport. While there is a PAX paid shuttle service it was easier and cheaper to catch the local bus which had a stop 100 meters away. Just used my iPhone contactless payment. The next morning I took it back to the domestic terminal then immediately caught the shuttle over to the international terminal. While I did not wait, another pax noted they have been waiting about 20 minutes for the shuttle.