The world's coral reefs are at a "make or break point" and every nation should think twice about developing new coal mines, including those planned for Queensland, the United Nations environment chief Erik Solheim said.

Mr Solheim, in Australia on a whirlwind trip to meet coral experts and visit the Great Barrier Reef, said coral reefs were one of the planet's most important ecosystems, home to a quarter of the oceans' biodiversity in just 1 per cent of the area.

However, recent severe coral bleaching caused by warming waters was among their leading threats.

"Reefs are in peril," Mr Solheim told Fairfax Media at the start of the International Year of the Reef, aimed at lifting awareness of their plight.