ATSB director transport safety Kerri Hughes said the pilot reported an unusual noise as the helicopter was collecting water in a 1,200L bucket on a short line, followed by a “kick”.

“Remaining in the hover, the pilot observed all engine indications were normal and the bucket and line were in the appropriate place. However, concerned something was not right, they elected to dump the water and initiate a climb,” she said.

As the water was being released and the engine powered up, there was a “loud roaring sound”, and the helicopter pitched up, yawed, and lost power before rolling left and hitting the surface of the dam at low speed.

According to Hughes, the helicopter flipped upside down in the water and began sinking rapidly, and the pilot was able to undo their seatbelt and helmet but could not open the left front door with the normal or emergency release handles.

“When the helicopter was almost fully submerged, the pilot swam to the rear of the cabin and tried to open the rear right door but could not open it either, making further attempts to get out by kicking the helicopter windows,” she said.

“The pilot then moved to the rear left door and, utilising considerable force, was able to successfully open it.”

The pilot has credited the lack of seats in the cabin, as well as their familiarity with the helicopter and previous HUET (helicopter underwater escape training), as reasons they were able to successfully escape.

The ATSB investigation has so far involved interviews with the pilot and witnesses, as well as preliminary examinations of the wreckage.

“As this investigation continues, the ATSB will review and examine the pilot’s training and records, maintenance documentation, and key components of the helicopter,” said Hughes.

“Should a critical safety issue be identified during the course of the investigation, the ATSB will immediately notify relevant parties so appropriate and timely safety action can be taken.”