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Psilogramma casuarinae
A big fat caterpillar.....of the Privet hawk Moth. \
Backyard
I just checked again and I was wrong, the privet hawk moth does occur in Australia, sorry about that! You should add the common name and Scientific name when you get the time so that it's counted as identified on Noah.
I'm not sure that it's a privet hawk moth, those live in the Palearctic ecozone, which includes Asia, Europe, and Northern Africa. It does, however, look very similar to a few caterpillars we have in the US, in the genus Manduca. I would suggest looking for Manduca sp. that are native to Australia.