The plane crashed into a river in the Kimberley region on Sunday, killing all four people on board.
The aircraft was returning from a trip to Australia’s Gold Coast when it ran out of fuel.
The plane had taken off from Sydney at 8:30am local time on Saturday, heading for Darwin.
The National Transport Safety Bureau (NTSB) said in a statement that it was looking into the cause of the crash.
It is not yet known how many people were on board the plane.
The NTSB said that the crash occurred after the aircraft hit a rock about 50 metres from the shoreline.
“The aircraft was operating with a low speed of 5 km/h, and the aircraft’s flight crew did not make any manoeuvres to reduce its speed and maintain a safe level of speed,” the NTSB added.
The pilot was identified as an Australian of Indonesian descent, the NTSb said.
“He was not on duty at the time of the incident, and did not receive any direct communication from the aircraft,” the statement said.
The crash site is in the middle of the river where the plane hit.
The Federal Government has been working on an overhaul of the aircraft, which has a range of about 1,500km.
The Australian Transport Safety Board said that it had not received any information from the pilot’s crew on the cause or cause of death.