Thousands of blue jellyfish-like creatures are spoiling the dream of a seaside long weekend for holidaymakers on the West Coast of the lower North Island.

Swarms of the creatures known as by-the-wind-sailors, or velella, have been spotted on the sands of Paraparaumu Beach this morning following reports of the sea life appearing at beaches across the country's West Coast for the past few weeks.

A Paraparaumu resident contacted the Herald with photos of the "colourful visitors" and said there would be no swimming for them over the public holiday.

The sighting is one of many that have been reported around New Zealand since the start of October, when the creatures first started appearing at beaches in the Nelson, Kapiti Coast, Taranaki and Greymouth regions.

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A close-up view of the velella hydrozoans, which are also known as by-the-wind-sailors.
A close-up view of the velella hydrozoans, which are also known as by-the-wind-sailors.

DOC marine expert Don Neale told the Herald by-the-wind-sailors were related to jellyfish and "bluebottles", which also often washed up.

They are said to have got their name from the small erect "sail" on their backs, which allows propulsion by the wind.

"They're quite often seen washed up on West Coast beaches and when there's one there's usually dozens, but I've never seen them drifted up this much (in their thousands) in one place before," Neale said in an earlier article.

"As the sea starts to warm up in the spring, all the floating plankton starts to bloom and it's not unusual to get a lot more things like this starting to wash up, as well as 'red tides' of plankton out at sea."

Velella have washed up on Paraparaumu Beach.
Velella have washed up on Paraparaumu Beach.

The first mass stranding of by-the-wind-sailors was recorded on the North Island's west coast in 2006. Scientists estimated 100 million velella were washed up on just one beach.

The strandings are thought to be caused as a result of the waves, tides, winds and currents.

By-the-wind-sailors are not considered dangerous to humans.