Now we know the answer. It's a caterpillar, the Sheoak Moth caterpillar Pernattia pusilla. They are in plague proportions and are defoliating the sheoaks. If you drive the roads between Geelong and Barwon Heads or walk creek reserves you'll see a lot of seemingly dead sheoaks.
With this in mind I checked the male Drooping Sheoak growing in my garden ... and found one caterpillar at eye level. Hopefully he is a loner. The tree is looking pretty spectacular at the moment, covered in long cascades of male flowers.