Why on earth is a police investigation underway..
It is standard procedure in Oz for the State Police (or the Feds in major ports once they've arm-wrestled control from the Staties) to assume command in any aircraft emergency, so as a Mayday was declared this would be following the protocol.
I'm not sure of the demarcation for any following investigation, but presumably the ATSB (Aviation Transport Safety Board) will take it on at some stage.