Black-spotted Whipray

Found singly, on soft bottoms of lagoons, bays and protected areas of shallow reefs, also deep on the continental shelf.
Western Pacific - Australia, Papua New Guinea

Whiprays generally have much longer tails.

Rays are bottom feeders, they settle themselves down over their prey, trapping them against the substrate with their disc, then flexing their disc flaps and manoeuvring the victims into their mouths.
Most rays live in the sea, but some can be found in estuaries, often hard to see as they can be buried in the sand, occasionally they can be seen leaping out of the water.
As a form of defence rays have electric organs while others have venomous spines.
Usually rays will swim out of harms way if approached, however they can give a nasty sting which could prove fatal.
Never swim over the top of large rays, they think you are a predator and are likely to whip their tails in defense! (as happened to Steve Irwin)