A flotilla of bluebottles has washed ashore on Sydney beaches creating a beautiful, but dangerous lining on the sand.

Australian Museum naturalist Martyn Robinson said the bluebottles' arrival was a sign Sydney waters were warming.

"During the past few months the water has been quite warm and the winds from the north-east blow them onshore," he told Richard Glover on 702 ABC Sydney.

The little blue beast found on Sydney beaches is most commonly a Pacific Man o'War, which can grow up to 15cm.

The larger bluebottles which tend to dwell in the Atlantic Ocean — Portuguese Man o'War — also occasionally wash into Sydney waterways.

Man o'Wars are found—sometimes in groups of 1,000 or more — floating in warm waters throughout the world's oceans

Researchers have found hot water is the best way to treat a bluebottle sting. This bluebottle washed ashore on Dudley beach, near Newcastle. ( Supplied: Shellie O'Meara )

Bluebottles are not capable of swimming ashore and once beached will remain on the sand until the waves wash them away, creating a kaleidoscope of colour on the sand.

"They have a little float on the top which they can angle to catch the wind," Mr Robinson said.

But swimmers have been warned bluebottles could sting, even when washed ashore.

"They can even sting you after the animal itself is essentially dead," Mr Robinson said.

And the stinging cells in the long blue trailing tentacles could still fire a barb, even if the bluebottle "head" had broken up in the waves.

More than 10,000 people report being stung by bluebottle in Australia each year, with up to 30,000 reports in peak years.

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Hot water the best treatment

Remedies for treating bluebottle stings in the past have included ice, vinegar, urine, cold tea and warm beer.

Recent Sydney University research found hot water to be the best treatment for bluebottle stings.

"One report suggests you immerse the stung area in as hot water as you can stand; not hot enough to cook the flesh, but hot enough to denature the venom," Mr Robinson said.

Bluebottles entangled in seaweed on Pearl Beach. ( Supplied: Ian Lisser )

He advised to remove any tentacles carefully with tweezers or a gloved hand before applying treatment.

"The main thing is you do not use alcohol or vinegar which can make [the stingers] fire again," he said.

But despite their sting to swimmers, bluebottles do have a purpose in the sea.

They provide a food supply for sea lizards and sea turtles, who can eat the pest without being stung.