So excited are the specialist reef folk from James Cook University, they have dubbed the event the "world's biggest orgy".

It was, according to Associate Professor Jamie Seymour of the Cairns-based Australian Institute of Tropical Health and Medicine, one of the most spectacular events he has seen in 30 years of studying the reef.

"Visually it was mind blowing," he said.

"I've seen it like that once before about 15 years ago but nothing as spectacular as this.

"Everything aligned, the water visibility was spectacular, there was no wind, no current so as it happened, everything floated to the surface, rather than being washed away.

"Think of it as standing in a snow storm with little snow crystals going everywhere.

"When everything floated to the surface, with 20 metre visibility you could sit on the bottom and look at a layer of eggs the on surface of the water."

Associate Professor Seymour said the rarity of the event stemmed from the majority of the reef coral spawning in less than two hours and on one night, in addition to the perfect storm of conditions that made it such a visually spectacular event.

He said coral usually spawned over three to four hours over three to four nights following a full moon in November or December and it was unusual for such a diversity of species to all go on the same night.

He said often the spawning was spread over both November and December full moons, depending on at what time in the month the astronomical event falls, with water temperature critical to the process.

He said the event pointed to the reef being in good health.

"When things are spawning, when things are breeding and breeding, well you have to think the animals are in good shape," he said.

"If you are stressed and things are looking sad and ugly, you are not going to spawn as spectacularly as they did last week.

"It really is one of the natural wonders of the world to see this happen."