The hard, reef-building corals of Australia’s Great Barrier Reef belong to the order Scleractinia.
Scleractinians are commonly called stony corals because they produce rigid skeletons made mainly of calcium carbonate. Over many generations, these skeletons accumulate and form the physical framework of coral reefs. Most reef-building corals on the Great Barrier Reef are colonial animals made up of many tiny individual polyps.
Scleractinia is an order within Anthozoa, the broader class that also includes sea anemones and soft corals. Anthozoa is therefore too broad to be the requested answer. Alcyonacea, meanwhile, contains soft corals and octocorals, which generally do not create the same massive limestone framework as stony corals.
Modern scleractinians include both reef-building and non-reef-building species. Their success depends on factors including sunlight, temperature, water chemistry, and the partnership many species maintain with Symbiodiniaceae algae. This combination of animal biology, skeleton production, and algal symbiosis enables large reef systems to develop.