"The run-off from torrential rainfall goes into the Great Barrier Reef lagoon and straight into the ocean at speeds which were not thought to occur before we saw the images," said Arnold Dekker, from the Commonwealth Scientific and Industrial Research Organisation (CSIRO).

Scientists say global warming is already a major threat to the Great Barrier Reef, which stretches 2,300 km (1,400 miles) down the northeast coast, with rising sea temperatures expected to cause more frequent coral bleaching.

Global warming is also expected to result in more frequent storms, such as cyclones that lash Australia's tropical north and flood rivers flowing into the Great Barrier Reef lagoon.

The satellite images, of a 100 km (62 miles) stretch of reef off the tropical city of Cairns, show sediment from rivers recently flooded during a cyclone. The water was travelling at one kilometre per hour onto the reef, Dekker told Reuters.

"The conventional wisdom was that this kind of water would stay quite close to the coast ... and would slowly diffuse across the Great Barrier Reef," he said.

"What these images are showing is that the plumes of river water would go straight through the reef out into the outer reef, which is something which we hadn't seen before."

Dekker said the sediment run-off also carried pesticides washed off farmlands, which might threaten the reef's ecology.

"The satellite images seem to indicate that it is diluting quickly and that after a week the direct effects are gone," Dekker said.

"However, we don't know how resilient the outer reef organisms are to the large body of sediment which is re-suspended all the time with wave and current action," he said.