Processionary caterpillars (Ochrogaster lunifer). You can see their unusual nest in the last 2 photos.
Found throughout coastal and inland Australia, they feed on acacias and beefwood, and very occasionally on eucalypts, and they display a fascinating behaviour that helps them find new sources of food. Once these caterpillars have stripped a plant of its edible foliage, they will wander off to find more, leaving a silken trail behind them. Processionary caterpillars instinctually follow these silken trails, so it’s not uncommon to see groups of up to 200 of them moving head-to-tail in a tightly formed line, seeking a fresh supply of food at the other end. Also kinda cute, but also a bit gross, is the reason these insects are nicknamed bag-shelter moths. Large numbers of these caterpillars live together in a bag nest made from silk, excrement, skin particles, and other debris, which they leave periodically in search of food, until they’re ready to pupate and transform into their moth form underground. ~ Australian Geographic.