These pesky but striking caterpillars usually appear in great numbers and can strip a small eucalypt bare of all it’s leaves. They are the laval form of a fairly unremarkable brown moth.
These infestations were photographed on a white flowering Spotted Gum (Eucalyptus maculata), and in previous years were found on an adjacent Yellow Bloodwood (Eucalyptus ximia) which they stripped bare.
The thorax has four spikes with stinging hairs so another species to approach with caution. More information on the Black Slug Cup Moth can be found at http://www.brisbaneinsects.com/brisbane_loopers/BlackSlug.htm.