Leonardo DiCaprio, star of The Great Gatsby, says concerns about Australia’s Great Barrier Reef have raised his environmental awareness.

“Since my very first dive in the Great Barrier Reef in Australia 20 years ago, to the dive I got to do in the very same location just two years ago, I’ve witnessed environmental devastation firsthand,” DiCaprio said.

“What once had looked like an endless underwater utopia is now riddled with bleached coral reefs and massive dead zones.”

DiCaprio appeared at the Our Ocean Conference in Washington DC where a video message from President Barack Obama was played.

US Secretary of State John Kerry also attended.

President Obama plans to ban fishing, energy exploration and other activities in a large swath of the central Pacific Ocean, creating the world’s largest marine sanctuary.

This would expand about 225,000 square kilometres to more than two million square kilometres in the US-controlled Pacific Ocean area between Hawaii and American Samoa.

Last year DiCaprio announced a $US3 million ($A3.25 million) donation to help protect the oceans’ habitats for marine species.

“It’s fantastic to start off the day by hearing President Obama commit to expanding marine reserves in US waters and taking serious steps to prevent illegally caught fish from entering the marketplace,” DiCaprio said.

Read more on State plans to boost Great Barrier Reef tourism.