The colour, pattern, and proleg arrangement is characteristic of Plusiinae. It would be interesting to know on what plant it was feeding, and it's species of you can rear it to an adult moth?
Plant is most likely (99% sure) Conyza albida. Also known as tall fleabane, grows up to 2 m high (yep they grow that tall at my place if I don't pull them out first ... or something doesn't eat them) and not an Australian native. Something has vanished most of the caterpillars although I did find one that was beginning to pupate so theoretically have it safe. No guarantees.