"We're seeing huge levels of bleaching in the northern thousand-kilometre stretch of the Great Barrier Reef."

Of the 520 reefs he surveyed, only four showed no evidence of bleaching.

From Cairns to the Torres Strait, the once colourful ribbons of reef are a ghostly white.

"It's too early to tell precisely how many of the bleached coral will die, but judging from the extreme level even the most robust corals are snow white, I'd expect to see about half of those corals die in the coming month or so," Professor Hughes said.

I'm really angry that the government isn't listening to us, to the evidence we've been providing to them since 1998.

Reef scientist Justin Marshall

Professor Hughes said he is frustrated about the whole climate change debate.

"The government has not been listening to us for the past 20 years," he said.

"It has been inevitable that this bleaching event would happen, and now it has.

"We need to join the global community in reducing greenhouse gas emissions.

"For me, personally, it was devastating to look out of the chopper window and see reef after reef destroyed by bleaching.

"But really the emotion is not so much sadness as anger.

"I'm really angry that the government isn't listening to us, to the evidence we've been providing to them since 1998."

Mr Hunt told 7.30 that he was confident in the advice from the Great Barrier Reef Marine Park Authority — that the southern and central parts of the reef had so far escaped serious bleaching.

He said the Government had committed $2 billion over the next decade to protect the reef through initiatives such as improving water quality and removing the crown-of-thorns starfish.

A spokesperson for the Great Barrier Reef Marine Park Authority said it would be following up the aerial surveys with in-water surveys over the next two weeks to determine the true extent of the coral bleaching.