It's not clear from the reports that they were technically cave diving, they were in the sink hole but I don't think that automatically means they were in an overhead environment?
I can't comment specifically about this incident nor the people involved, but here is some information about what I know of the site from my experience.
This site is on private property, and access to it is strictly controlled by the CDAA (Cave Divers Association of Australia). Generally only CDAA members are allowed access, with some special considerations for visitors with equivalent qualifications).
I believe the last death at this particular site was in 1972 or 73, and was one of a number of deaths in the area during a short period leading to the inception of the CDAA.
The site is classified as a sinkhole. It widens out considerably as you descend, and is considered an overhead environment. Descending directly from the entry point, the bottom is at ~20m (60'), and depending which direction you take from there can get to as much as 60m (180'). There are two fixed lines running down to about 40m (120'), and can be useful for orientation in the hole, but not required for navigation. The water is without fail crystal clear.
This picture was taken from 20m, looking up towards the surface. The rock formation you can see is actually above ground, about 15m above the surface.
There are a lot of large boulders creating wide swim-throughs of 2m - 5m (6' - 15'), but also along the walls there are a handful of longer, tighter tunnels that could be classified as caves, silt up easily, and require running a line through.
The one pictured below runs from about 27m (100'), and gets to about 42m (127') before exiting at 37m (120'). So if a diver were to get entangled in that passage, there is not a lot of space for a buddy to assist, and not a lot of spare time to get out.
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