It appears the Ozzy ones are large buoys with trailing bits at a few various depths anchored in deep water with wicked currents on the east coast above Sydney I think.
The one in NP had a large buoy with a flag on top, I don't know what hung or trailed under water.
I don't know anything about about the one off the Knights.
The home made one in 110m off the chicks was about 10m below the surface.
As to shipping/yachts/launches ... all vessels. I have worked on several Navy Hyrdrographic Survey ships, so I have seen chart corrections, up date to mariners charts and so on being created and then distrubuted... which these day will come through with software updates, thus the area on a chart will have a hazard warning drawn around it... but yeah, this is not a fail safe method for some twat driving into it that hasn't planned his navigation properly.
"The sea, once it casts its spell, holds one in its net of wonder forever." - Jacques Cousteau