Sawfly (perga spp.) Two possibility: Perga affinis or Perga dorsalis. In "Pests, Diseases, Ailments and Allies of Australian Plants" book by David Jones (2015), page 284 and 285, Perga dorsalis is longer (up to 7cm) than Perga affinis (up to 4cm), and has prominent bristly white hairs.
This sawfly larva appears to have newly moulted the top half of its body, while the rest of it is still black, as are the other larvae nearby. I thought it looked so unusual, propped up like a little teddy bear above the black bodies.