An Air Transport Safety Bureau investigation was immediately launched into the cause of the crash with three investigators arriving in the area on Sunday afternoon.
ATSB spokesman Stephen Curry said investigators would assess the crash site and wreckage to understand how the plane struck the water and conduct interviews with the pilot, surviving passenger and any other witnesses.
They would also retrieve the wreckage, study engine systems for possible faults or failures, and check for any other evidence suggesting what may have caused the crash - although it was unknown whether or not there was any "black box" style device on the plane.
A preliminary report would be released in about 30 days, with a final report taking as long as 12 months.
A REPORT will be prepared for the information of the Coroner following a fatal plane crash at Ewingar this afternoon that killed a 11-year-old girl.
About 12.45pm today police and emergency services were called to Ewingar Ridge Road, Ewingar, about 85km south-west of Casino, following reports of a plane crash.
It's believed the aircraft hit power lines and then crashed into a nearby river about 11am.
The male pilot, aged in his 50s, suffered minor injuries, but managed to free himself from the wreckage and alert authorities. He has been taken to Lismore Base Hospital for treatment.
A 34-year-old male passenger was later released from the wreck and taken to Lismore Base Hospital by helicopter. He is in a serious condition.
A 11-year-old female passenger died at the scene.
A crime scene has been established and will be examined by specialist forensic officers while an investigation into the cause of the crash will be undertaken by the Australian Transportation Safety Bureau.