Somewhat ironic and maybe even a pointer, John Beans great photos of Lake Eyre ominously show an aircraft wreck on the salt lake, with the footnote... "The plane was put into the lake in the flood of 1990 when the pilot lost his horizon in the blurred reflection of water and sky."
Journey to the Lake Country
I note that the area where the chopper went down is extremely marshy, with the first party on the scene having to access the crash site by boat. Areas of shallow water and moonlight can lead to nasty eye tricks.
The other side of the ledger is that Gary had been flying for 40 years, in a high-stress job, and was old enough to have the possibility of a hidden medical problem.
A stroke or heart attack will incapacitate you in seconds, and there was no-one alongside Gary who could grab the controls and recover, if he'd suffered sudden medical incapacitation.
The only thing we know is that the chopper flew into the ground at high speed, and that can only mean sudden and severe mechanical failure, sudden pilot incapacitation, or the pilot losing orientation.
Loss of orientation must be relatively low on the list, due to Garys extensive experience. He must surely have encountered similar situations hundreds of times, and would have known exactly what the score was, in those conditions.