Hippotion celerio (Linnaeus, 1758) Gabi Moth, Vine Hawk Moth
You must have disturbed it as it has tucked its head under its thorax and expanded its eye segments. It's agriculturally important as the adult moth contributes to the pollination of Papaya.
Interesting question. It appears to be a "horned" caterpillar, similar to the tomato hornworm. It would be helpful to have any more information you have, such as area found, was it found on a plant and if so what plant? The associated plant will be an important piece of information; tomato hornworms are laid on a host that they can benefit from, and then stay on that plant until they absolutely have to move, and then relocating by the minimum distance possible. So it should have been found very close to the host unless it was dropped by a predator.
There is a site dedicated to identifying hornworms in Australia at http://lepidoptera.butterflyhouse.com.au/faqs/ident.html
Edit: this could be close - Vine Hawkmoth. See https://www.whatsthatbug.com/2011/05/14/vine-hawkmoth-caterpillar-from-australia/