This is an amphipod (order: Amphipoda) about 8mm, common under logs in damper areas. I see them most often in Namadgi but have also seen them in Black Mountain reserve and Bruce Ridge.
When first exposed they flip all over the place with a similar movement to a prawn out of water and then disappear into small holes or under loose dirt in a matter of seconds. They are extremely difficult to photograph due to this habit, which is probably why such a common animal is not reported. I also seen amphipods under rocks along the Molonglo and Murrumbidgee rivers although its possible they are a different family or species.