This is an example of a mature caterpillar. They're born black with white thorax and long, sparse grey hairs. When mature, they turn brown with tufts of grey/white hair on the sides between segments.
This one was crawling fairly rapidly across one of my outdoor chairs. The species seems to be specific to Australia.
I've always known these caterpillars as "spitfires", but I don't think that's right. If anyone has any more information, I'd be grateful for it :) Here's another site for reference: http://www.brisbaneinsects.com/brisbane_...