Great photo. There is a published photo of a Geometrid caterpillar that has a red mark on the thorax: Oenochroma pallida http://lepidoptera.butterflyhouse.com.au/chro/pallida.html but Steve's photo there does not show any fine white spots, like those that are visible all over your specimen. It could be a different instar, but more likely it is a species for which we do not know the caterpillar yet. Of the 1,270 named Australian species in Geometridae, the Coffs Harbour Butterfly House website only has larval pictures for 147.