I received the following email today
Thank you for sending your image through to Discovery Centre at Melbourne Museum.
I have shown the image to a Lepidoptera expert who visits the collections each week and he has said the larva is a species of moth and not butterfly and that there are several different families that it may belong to. If you are really keen to know and if you can get it to pupate and have the adult emerge we are more than happy to look at the image of the adult. If you are willing to continue to sacrifice some silver beet feed it until it pupates, put the specimen in a jar with air holes and take a photo of the adult when it emerges. Just be careful in handling any hairy caterpillars or their cocoons as they can be capable of causing irritation to people.
Even the experts can't tell!
Such a great service the museum provides and I will definitely be trying to get a picture if all goes well. See my earlier posts on our new pet for more on our mystery caterpillar http://landcare.blogspot.com/2011/09/eating-rainbow-and-caterpillar.html