Australia pledged another Aus$5 million (US$4.6 million) to the fight against a predatory starfish devastating the iconic Great Barrier Reef Thursday, revealing 100,000 of the creatures had been wiped out so far.

Environment Minister Mark Butler said the new funding, on top of Aus$2.53 million already pledged, would support a programme of culling the coral-eating crown-of-thorns starfish, which is naturally-occurring but has proliferated due to pollution and run-off.

A major study of the reef's health published last year revealed coral cover had halved over the past 27 years and attributed 42 percent of the damage to the starfish.

Canberra formally downgraded the reef's health from moderate to poor last week, with cyclones and floods depleting water quality and reducing coral cover by 15 percent since 2009.

The government is under pressure to improve conditions on the reef, with UNESCO warning its World Heritage status will be declared at risk next year without action on rampant resources and coastal development in the region.

Butler said a special programme to reduce starfish numbers launched last year had seen 100,000 crown-of-thorns eradicated and the extra funding pledged Thursday would support a dedicated culling boat and divers.

"Importantly, it means crown-of-thorns starfish on high-value reefs are prevented from entering the next spawning season, and coral cover at high-value tourism sites, such as Lizard Island, has been maintained," Butler said of the progress made so far.

He said action on the starfish, which consumes coral faster than it can be regenerated, was urgent given the broader threat of global warming to the reef.

"Due to climate change, the incidence of extreme weather events have had an incredibly detrimental effect on the reef," Butler said.

"Also, since 1979 we've seen devastating coral bleaching occur across the reef nine times due to climate change and our warming sea waters, when there was no previous recorded occurrence."

Butler said tourism and related reef activities injected Aus$6.2 billion into the economy every year, employing 120,000 people and it was one of the nation's "most valuable assets".

"We must ensure we protect the reef and the jobs it supports, which is why acting to halt climate change and further damage to the reef, by cutting carbon pollution, is imperative," he said.