In the daylight it's simply an unappealing layer of pink slime floating on the water.

But at night the "discoloured smear" on parts of the waterfront in Wellington transformed into a dazzling, glowing blue, caused by bio-luminescent organisms that washed up near the marina.

Wellington woman Sarah Day this week captured video of the stunning effect down at Chaffers Marina - but warns other Wellingtonians not to come looking for it.

"If I was looking for it I would look in areas between things where the water is a bit protected from the waves," she said.

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By now the slime had dispersed from where she found it in the marina, but regardless, it was found in a private area that the public don't have access to.

"In a southerly wind it accumulates on the south side of the pier, in behind boats that protect it from being blown away immediately," Day said.

The slime was "around quite a lot" in various places on the waterfront, but not usually as thick as when Day filmed it the other day.

"[It] looks like a discoloured smear on top of the water during the day."

It can most often be spotted after settled weather, with sunny days.

"The winds and waves move it around a lot."

Day said the bio-luminescence was "pretty magical" and appeared much brighter in person than it did on videos or in photos.

According to Te Papa, bio-luminescence occurs at any depth of the ocean and is caused by living organisms experiencing a chemical reaction.

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The reaction happens when a substance called a luciferin is oxidised, and the most common coloured reaction is blue.