I think it is an Australian Painted Lady caterpillar, I photographed one near the spot where I found this caterpillar last week. http://landcare.blogspot.com/2011/09/bees-and-butterflies.html I found it eating my silverbeet and despite offering it alternatives it is sticking to its favourite. I have been reading a number of U.K. blogs and this seems to be more of a done thing over there. Our first time keeping a caterpillar indoors and we're thoroughly enchanted, hope it turns into a butterfly.
Apparently less than 5% of butterfly eggs make it to maturity (surely this is not true with those cabbage whites), a lot of predators of the poor old caterpillar out there.
More on caring for caterpillars here