Environmental groups Monday urged greater action on climate change after the government sounded the alarm over severe coral bleaching in the pristine northern reaches of Australia's Great Barrier Reef.

The government said Sunday that corals had turned white and grey in parts of the World Heritage-listed marine park, with the bleaching "severe" in northern areas.

Environmental group WWF said large sections of coral near Lizard Island were drained of all colour and fighting for survival.

"The reef can recover but we must speed up the shift to clean, renewable energy and we must build reef resilience by reducing runoff pollution from farms and land clearing," said WWF spokesperson Richard Leck.

Bleaching occurs when abnormal environmental conditions, such as warmer sea temperatures, cause corals to expel tiny photosynthetic algae, draining them of their colour.

Corals can recover if the water temperature drops and the algae are able to recolonise them.

The Great Barrier Reef Marine Park Authority said recent underwater surveys had detected "substantial levels of coral mortality" in the remote far north areas, blaming prolonged higher than average sea surface temperatures.

As a result, it has raised its response to level three -- the highest level in its response plan and indicating "severe regional bleaching".

"The pictures we're seeing coming out of the northern Great Barrier Reef are devastating," said Greenpeace Australia Pacific's Shani Tager.

"The Queensland and federal governments must see this as a red alert and act accordingly."

She called on the government to reconsider coal mining, saying the burning of the fuel was "driving climate change, warming our waters and bleaching the life and colour out of our reef".

Scientists had feared that the current El Nino weather phenomenon -- when the trade winds over the tropical Pacific start to weaken and sea surface temperatures rise -- would impact the reef.

One of the worst mass bleaching episodes on record, which affected reefs in 60 tropical countries, took place in 1998 when the El Nino phenomenon was exceptionally strong.

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But the independent crowd-funded Climate Council said while El Nino events had been experienced before, such severe bleaching would not occur without the influence of climate change.

- Wake up call -

Environment Minister Greg Hunt, who inspected the area by air on Sunday, agreed that despite periodic El Ninos, overlaying such events with climate change "does exacerbate them".

"I don't think there's any debate in the scientific community on that front. That's the advice of all of the marine scientists and climatologists with whom I've worked," he told the Australian Broadcasting Corporation on Monday.

"And that's why the Paris outcome... is fundamental," he said, referring to the historic global deal agreed by 195 nations last year aimed at curbing carbon emissions and limiting warming.

Hunt said there were some positives, with experts saying the bleaching was nowhere near as bad as in 1998 or 2002, and with three-quarters of the reef experiencing "minor to moderate bleaching".

Russell Reichelt, chairman of the Great Barrier Reef Marine Park Authority, said the arrival of the wet season had also appeared to have spared most of the vast 344,400 square kilometre (132,974 square mile) marine park.

But Jodie Rummer, a senior research fellow at James Cook University, said the situation was "not good at all" at Lizard Island in the north.

She said while the northern parts of the reef were among its most beautiful and pristine, they had also been hard hit by cyclones in recent years which had caused structural damage to the coral.

"It's quite sobering as well to think that this is the wake up call that we're getting to take better care of our environment," she told AFP.

Rummer said it would be weeks before the full extent of the bleaching was known.