Vortex Ring
Vortex Ring is the apparent culprit here. Some may call this settling with power; it is the same phenomenon.
If a helicopter is flown on a downwind approach with a high sink rate and low forward speed it will assuredly enter a vortex ring state.
The result is always thus; the descent will continue until the pilot reduces collective and increases airspeed, or where neither airspace or altitude allow, the helicopter must crash.
The reports that I have seen on TV appear to say that the pilot was ordered to make that approach. I doubt that, for he must know that to do would constitute a kamakazi operation. No Australian pilot is going to be so mute - or obedient!
Sorry to say this: pilot error, it seems. There but for.....etc
The Kite