The cchange erpillrrs seem to change a lot with age. I'm not sure, but this caterpillar looks quite mature and is similar to photos of Euproctis edwardsii I can see online. See: http://lepidoptera.butterflyhouse.com.au/lyma/edwardsii.html
@ibaird thank you, they were on a gum tree and Euproctis looks like it feeds on mistletoe. I’m not sure they are all the same species as there were a bunch that were large and a bunch that were tiny - nothing in between.