Probably H. armigera, but we only know the caterpillars of 15 out out of the 42 named Australian species in Heliothinae, so it could be one of the other 27. If you are keen to discover its species, you will have to rear it in protective custody (most caterpillars are predated) to get an adult moth.
Thanks Donhe - Your help and knowledge is appreciated - I'm learning all the time! It's moth will have to remain a mystery as I returned it to the garden after I'd taken my pics!
Our knowledge of Australian caterpillars is so incomplete. It would be good if you could always carry say a little plastic specimen bottle when you go out, to collect for rearing, any caterpillars you encounter.
I'd need to research how to house and look after them etc. as I've never done anything like that before! I think I need to give it a go, it would be very interesting! Thanks Donhe!