A swarm of stinging bluebottles has surprised beachgoers by washing up at a popular Northland swimming spot.
Kerikeri woman Francesca Jago spotted the strikingly blue creatures when she went for a paddle at Matauri Bay on Wednesday.
As she emerged from the water, just after low tide, bluebottles were washed up about a metre apart as far as she could see in either direction.
Originally from Australia, Jago is all too well acquainted with bluebottle stings — ''they hurt!'' — but was surprised to see the subtropical creatures in such numbers in New Zealand while the sea was still cold. The water temperature was just 16 C, she said.
Jago didn't see any of the sea creatures last year despite frequent visits to the beach.
Niwa researcher Dennis Gordon it wasn't water temperature but sunlight hours that mattered to bluebottles because that influenced their food supply.
The water was still cold but longer daylight hours meant single-cell plant plankton was thriving.
''They respond to light and once they take off that feeds the next layer up the food chain.''
Plant plankton was eaten by zooplankton which was eaten by fish, both of which were food for bluebottles.
Gordon said bluebottles were completely at the mercy of winds, tides and currents, which determined where they washed up.
The bluebottle, or Indo-Pacific man o' war, is not a single organism like a jellyfish but a colony of separate, specialised polyps which work together as one.
One polyp forms the float while others are responsible for catching prey, digesting food and reproduction.
They hunt by entangling small fish in their tentacles, which can grow more than 10m long, then paralysing them with stings.
Onshore winds regularly blow huge swarms on to beaches in Australia, where thousands of bluebottle stings are reported every year. The stings can cause significant pain and swelling.
Jago said she was planning to hit the water again on Sunday, ''but I'll be looking out for bluebottles''.
Ministry of Health advice for treating bluebottle stings:
■ Flush the stung area with sea water (or fresh water if sea water is unavailable) to remove the tentacles. If tentacles are still attached use a dry towel to remove them. Wear gloves if you have some.
■ Immerse the area in hot tap water for 15-20 minutes. Make it as hot as the person can bear without causing burns (no more than 45C). The heat will break down the toxin.
■ You can repeat the immersion for up to two hours but for no more than 15-20 minutes at a time with breaks between so the skin can cool.
■ Take pain relief.
■ Don't apply vinegar because it can make the sting worse. Vinegar works only for stings from the Australian box jellyfish.
■ Call your doctor if you've taken the steps above but you have increasing numbness or difficulty breathing, signs of poisoning (abdominal pain, cramps, nausea, vomiting) or, later on, signs of infection (increasing pain, redness, swelling, red streaks leading away from the sting, heat, pus, fever or chills).