AIUI The Great Barrier Reef is a living coral reef which grew under water. I am not sure if the sea level changed twice - once to reveal it to become the coast of australia, and once to return it to the sea - or maybe the programme was referring to a larger area of continental shelf being above sea level, with its edge approximately coinciding with the reef? I'm interested in knowing more but the linked paper doesn't seem to be downloadable.

This is a good visualization: http://www.bluehabitats.org/wp-content/uploads/2015/04/Scree...

The continental shelf is quite flat. The reef follows exactly along the rim of the shelf. I believe the coast used to extend all the way to where the reef now ends. The reef started growing as this part was slowly submerged in shallow water. Coral reefs only build in the photic zome (<50m) since they (specifically, the algae which produce the nutrients that coral polyps live on) require photosynthesis.