The two main forms of reef on the GBR are barrier reef and fringing reef

Barrier reefs form on the edge of the continental shelf and have a distinctive shape. The side facing the open ocean is usually steep while the leeward side slopes gently. A strong current often flows between the reef and the mainland.

Fringing reefs form along the shoreline of the mainland or an island. Rising sea levels may form a lagoon between the shore and the reef.