Environmentalists have again warned of an ecological disaster at the southern end of Queensland's Great Barrier Reef, following the discovery of a dead dugong.

It was found washed up on a beach in Gladstone Harbour, the fourth dugong, along with three dolphins and 40 turtles that have been found washed up around the harbour since May.

Friends of the Earth spokesman Drew Hutton said he had seen first-hand the destruction around the harbour since construction of the LNG facilities had started.