The ATSB don't investigate every accident. They only investigate this sort of accident if there is some safety benefit to the wider aviation industry. By handing it over to the local police I would surmise that the eye witness accounts indicate that there is no safety benefit to be gained from tying up resources. There was also a Nanchang accident off the coast of Narabeen a couple of years ago and that wasn't investigated either. It should probably be investigated by the Warbird Association.