I've photographed very similar caterpillars and they have always been moths. Have a look at the Coffs Harbour BUtterfly HOue web site. http://lepidoptera.butterflyhouse.com.au/larvae.html Also if you look at Suzi's butterfly book, butterfly larvae are not as long as these caterpillars.
Thanks Alison, I'd certainly peruse Suzi's book for information if it hadn't fallen apart some months ago. Some critters have general rules for identification, I didn't realise length was used for determination within Lepidoptera but will try remember that from now on. Till now my basic rule was 'hairy and lairy' for moths and 'smooth and subtle' for butterflies; ironically the opposite of their mature states generally. Thanks again..
I have no idea what species of moth this is, but have suggested something to remove it from the butterflies and place it into the moths so a moth expert can look at it.
It appears to have only one pair of prolegs. All butterfly larvae have the full set of 4 pairs. Only 3 (out of over 80) families of moths have larvae with reduced prolegs, and only in one family, Geometridae, there are species with larvae with only 1 pair. The small pair of pale tipped dorsal horns near the middle of the abdomen, and the rather rectangular head, look diagnostic, but they do not seem to match any of the 163 Geometrid larvae photos (out of 1,270 named Australian Geometridae species) on the Coffs Harbour Butterfly House website, see the webpage link on http://lepidoptera.butterflyhouse.com.au/geometridae/geometridae-cats.html
So to determine the species, it needs to be reared to the adult moth, as there are web-published photos of the adult moths of most of those 1,270 species.
Aha: found a match: Monoctenia falernaria has the pale tipped dorsal horns and the rectangular head, although we do not know what the larvae of the other Monoctenia species (Monoctenia smerintharia) look like, so perhaps we should be more conservative and just suggest Monoctenia species till we know that ?