Thanks to Marcus Leach for the initial sighting of this very active caterpillar in the middle of a road. It was constantly moving the head end around in all directions and didn't appear to be making any progress in a particular direction.
It could be mature and seeking somewhere nice to pupate. Many Geometrids burrow under the soil to pupate, and the ground your beastie was on looks very frustrating for that activity.
Did you take it into protective custody? Basically the only way to determine the species of most caterpillars is to rear them in protective custody (to keep predators and parasites away), and rear it to the adult moth. Otherwise it is a useless observation.
Thanks for the identification to date and apologies for the useless observation. Whilst I understand the issue, a lot of the images I post are taken in the Australian National Botanic Gardens and as such the Gardens prohibits the public from removing any materials from within the Gardens. Consequently, I won't be removing anything from the Gardens, but will continue to post sightings which hopefully may contribute to CNM's records. Tim