BRISBANE, AAP – On Western Australia’s stunning Ningaloo coast, a coral bommie that began life 1000 years ago sits ghostly white in the green waters of Bill’s Bay.

Experts aren’t 100 per cent sure but they strongly suspect the death of the colossal coral community dubbed Ayers Rock.

It’s not a good sign that brownish algae is creeping over the structure, which rises three to four metres from the sea floor and is about eight metres across.

All around the ancient bommie, other corals have also bleached and are definitely dead, victims not of rising sea temperatures but of a coral spawning event gone wrong.

It was late last month, when the weather conspired against the synchronised, annual release of coral sperm and eggs in the bay, which is famous for its abundant reef system close to the shoreline.

Reef ecologist Frazer McGregor was lucky enough to be in the water when the reproductive spectacle unfolded.

He’s witnessed plenty of spawning events before but this one was profuse, leaving a thick soupy blanket on the surface of the sea.

“It was really dramatic … like a city with all the chimneys going off at the same time.

“Within half an hour we had to get out of the water because there was just so much spawn material and associated zooplankton. It was getting in our wetsuits, and all over us and it was quite smelly.”

Normally, it’s a trouble-free spectacle with the wind and ocean flushing out the bay and distributing newly created coral larvae while also getting rid of eggs and sperm that didn’t hook up.

But Cyclone Charlotte, followed quickly by a second low pressure system, meant that didn’t happen and unfavourable winds trapped the decaying material.

The result was a monumental stink and an oxygen-starved dead zone that killed not only coral but also enormous numbers of fish – as many as a million, Mr McGregor says – ranging from tiny damselfish to bigger species.

Stingrays and eels also took a hit.

It’s not the first such event to hit Bills Bay, offshore from the tourist township of Coral Bay. Locals remember others, every few decades or so. But it’s among the worst in recent memory, with a strip about 4km long and up to 300 metres wide hit.

Tom Holmes is a marine ecologist with WA’s Department of Biodiversity, Conservation and Attractions and says it could take anywhere from five to 20 years for damaged areas to recover.

“It’s really sad to see that effect on Ayers rock, in particular. It probably shows how significant this event was, in the scheme of things.”

But he says there are still plenty of healthy corals for visitors to see, and further offshore there’s no damage at all.

Mr McGregor, who runs a tourism business and manages the local research station, is choosing to focus on what comes next.

He says that if Ayers Rock is dead, its limestone bones will hopefully attract new generations of coral larvae.

“We’ve just had a full moon two nights ago so in another three to four nights there will be another minor spawning. It could get washed in as larvae and recolonise those areas. That’s the hope.

“How long that takes and what grows? It could be all the one species, it might not.”