Australian authorities said Sunday coral bleaching occurring in the World Heritage-listed Great Barrier Reef had become "severe", the highest alert level, as sea temperatures warm.

Australian Environment Minister Greg Hunt said while the bleaching at this stage was not as severe as in 1998 and 2002, also El Nino-related events, "it is however, in the northern parts a cause for concern".

"The reef is 2,300 kilometres (1,429 miles) long and the bottom three-quarters is in strong condition, but as we head north, it becomes increasingly prone to bleaching," Hunt said after an aerial tour of some of the affected areas Sunday, the Australian Broadcasting Corporation reported.

"Essentially what you could see was patches of coral bleaching as you approached Lizard Island (located in the Barrier Reef)."

The Great Barrier Reef Marine Park Authority said the highest response level, which it described as a "severe regional bleaching", allowed them to step up surveys to understand what was happening.

Hunt said the government would fund a survey of 40 sites in the reef area, which had been investigated in 2012, in September to assess the health of corals and potential recovery options.

"This information is particularly important for the future understanding of the reef given that the frequency of coral bleaching events and the severity of tropical cyclones are predicted to increase in the future," the minister said in a statement.

Bleaching is a phenomenon that turns corals white or fades their colours, threatening a valuable source of biodiversity, tourism and fishing.

It occurs when reef symbiosis -- the mutually beneficial relationship between two organisms that inhabit corals -- is disrupted by a rise in ocean warming, although there can also be other causes.

The reef -- the world's biggest coral reef ecosystem -- is already struggling from the threat of climate change, as well as farming run-off, development and the coral-eating crown-of-thorns starfish.

It narrowly avoided being put on the UN World Heritage in danger list last year with Canberra working on a plan to improve the reef's health over successive decades.