The bluebottle jellyfish, also known as Portuguese man o' war, were found in Campbells Bay, Kakanui, last week.
University of Otago marine science Prof Abby Smith said the creatures generally washed up on the west coast, but were carried by currents and offshore wind to beaches around the country, including Dunedin and Timaru.
They were typically found in tropical and subtropical waters.
To see them in the area was "not astonishing".
They also had large strandings on the east coast in 2002 and 2018.
MetService meteorologist Mmathapelo Makgabutlane said the transition from La Nina to El Nino was most likely the cause.
La Nina weather caused wind to come mostly from the north and north east, whereas El Nino had winds from the west.
Prof Smith said summer was the most common time of year to find bluebottles.
The days were long and warm, meaning there was plankton around for animals to feed on.
"The marine ecosystem is out there doing its thing."
She found the creatures very unusual.
"I’m always pleased to see them because they’re so interesting."
The bladder of the jellyfish could be touched, but the tentacles "packed a nasty sting".
People should be careful if they saw a bluebottle, as the tentacles could be buried in the sand or detached from the body and were still capable of stinging.
The sting could hurt for hours and the best thing to do if stung was to flick the tentacles off with an object and put the affected area in hot water, she said.
The welts left from the sting could last for a few days.
Some people could have an allergic reaction, which would require medical attention.
Dogs could also be affected and any that had eaten a bluebottle should be taken to the vet, she said.
If no jellyfish could be seen on shore or floating around there was no need to be concerned, she said.
