Water a clear blue-green, white caps on the swell – one of those days when it seems like the perfect place to be.

But a foul-looking surprise was lurking.

Over in the corner of the harbour, near the fishermen’s co-op, a mysterious slimy substance was spreading.

Bright green it was, thick and oozy, with a funny smell – but no-one knew its name.

One observer saw fish floating, lifeless, and other flotsam had accumulated too, drawn to it like being stuck in a thick slime.

Michelle Nolan was out for her morning walk and when she spotted the bright slick she was horrified.

Luckily, the spotter was a civic-minded citizen and she did not simply walk on by assuming it was someone else’s problem.

She called the ports authority and then the Environment Protection Authority. 

The slick was centred on a small fishing boat on the southeastern corner of the harbour.

The Mercury knows the identity of the boat’s owner and there is nothing to say it was the source of the slick.

Small crowds gathered to look at the bright green gunk, wondering what it was.

Some said it smelled like paint. Ms Nolan said one government agency she called told her it was blue-green algae. And it was smelly.

But while the public worried and wondered, Fire and Rescue NSW had already been out and tested the substance early in the morning after another person called in a report.

The good news is that there were no hydrocarbons in the substance – meaning it’s not oil or paint pollution.

Shellharbour Fire and Rescue station officer Murray Mackne was at the site at 8am on Thursday and said testing showed it was organic – it was a kind of marine life’s spawn, which often arrives on the tide after a full moon.

“It’s something that’s produced on a regular basis in the ocean,” he said.

“It’s a spawn from a marine organism.

“It’s not a toxic product – but it’s not a good looking product either, so you don’t want to swim in it … or ingest it.”

It was not blue-green algae, Mr Macke said.

So the mystery is almost solved. It’s spawn, it’s not toxic, but we’re not exactly certain what life form created the bright green bloom.