Thanks Andrew for your comments. I have just returned from a trip to Carnarvon Gorge, and tomorrow am off again for 2 months to Arfica and UK, so you won't hear much from me for a while.
Paul, I have just received a reply from the Qld Museum re the ID of the bugs, and as you said, it is a shield bug. Here is the email that they sent me.
Hi Mike
The four insects feeding on the orchard swallowtail caterpillar are predatory immatures of the Family Pentatomidae in the Order Hemiptera, or true bugs. There are at least 6 genera of predatory pentatomids in Australia.
See:
http://www.brisbaneinsects.com/brisbane_stinkbugs/PredatoryStinkBugs.htm
for some more information on this fascinating group.