The weather was perfect as we prepared to jump into the usually aquamarine waters of Lord Howe Island. But there was a strange pink and white slick that didn't look so inviting.

What was this gunk, that thousands of fish were tucking into?

Our snorkelling guide Caitlin Woods wasn't concerned. She was excited.

The slimy goo was the aftermath of a mass coral spawning — the rarely seen release of billions of coral eggs and sperm essential for coral to grow and regenerate.

We'd unknowingly snorkelled through a natural wonder, that was not pollution, but a sign of hope and life.

Coral sperm and eggs float through the water like a snowstorm. ( Dave Gardiner Photography )

The world's most unspoilt reef?

Lord Howe Island lies 600 kilometres off the east Australian coast, surrounded by the southern-most reef in the world.

The coral manages to survive this far south as the warm waters from the east Australian current wrap around the shores of a 7-million-year-old volcano.

"The reefs around Lord Howe Island are scientifically interesting because they contain a unique mixture of tropical species near their southern limits, some endemic subtropical species and some cooler-water temperature species near their northern limits," says Professor Peter Harrison, director of marine ecology and research at Southern Cross University.

Lord Howe Island is home to one of the world's most unique, intact reefs. ( Dave Gardiner Photography )

In other words, it's a pretty special mix of coral, that wouldn't usually be found at this latitude.

What's more, the coral is virtually unspoilt, 1000 kilometres away from the badly damaged Great Barrier Reef.

Caitlin studied marine science management at Southern Cross University before moving to the remote island three and a half years ago, intending to stay just a few months.

"I just fell in love with the place, Lord Howe has one of the most pristine reef ecosystems probably in the world these days, plus the terrestrial biodiversity is incredible."

Her passion is contagious and she spends most of her days guiding snorkelling tours for Greenback Marine.

The mass coral spawn only occurs once a year. ( Dave Gardiner Photography )

An annual secret tryst

While coral reefs engage in a mass spawning just once a year, the Lord Howe species has kept its midnight tryst a bit of a secret.

Few people have got the timing right and Caitlin failed for three years in a row.

"On a hunch I went for a night snorkel with photographer Dave Gardiner. As we headed out we saw all the spawn in the water and the corals releasing tiny pink and white balls."

"The balls were floating up to the surface like a reverse snow storm, it was incredible."

These packets of sperm and eggs combine to produce larvae; scientists like to say, "from little larvae big corals grow".

Caitlin described the mass coral spawning as a "reverse snow storm". ( Dave Gardiner Photography )

Reef health is fragile

But while the mass spawning is consistent with the healthy nature of the reef, Professor Harrison warns we must not become complacent.

"It's possible as global warming threatens reefs further north that Lord Howe will be even more important … but it is not immune from danger as we have recorded bleaching there too in 2010 and 2011.

"Ocean acidification could also tip the balance, changing ocean chemistry and making it harder for corals to secrete their calcium carbonate skeletons."

Hope for reefs

Caitlin prefers to look on the bright side and win the support of others to look after coral reefs:

"I think seeing the coral spawning phenomenon is an incredible beacon of hope … a reminder that things are continually regenerating here and as you look around the reef and see there are relatively few damaged patches it does make your heart swell with hope and optimism for the future of marine ecosystems around the world, knowing that an example of a pristine ecosystem can occur."

Lord Howe Island and its reef are on the World Heritage list and visitor numbers are restricted to a few hundred at a time.

"If we can help people understand a bit more about the underwater world here then I think they'll come to love it", says Caitlin. "And once they love it they will be motivated to conserve it."