He said they had been washing up on the West Coast as early as October 2.
But Neal said it was not uncommon for large numbers of marine creatures to be out in force.
"This sort of thing happens as the sea warms up in the spring time and into the summer and there's a lot of plankton blooming off the coast here around this time of year ... and that feeds the whole food web that these little jellyfish are a part of."
Neal said Velella had small stinging tentacles but were generally harmless.
"They shouldn't do any damage just touching them but if you do rub your eyes after handling them, that's when you could get into trouble, so it's just best to not touch them if you can avoid it."
Niwa emeritus researcher Dennis Gordon said the creatures would continue to accumulate and eventually rot on the beach instead of washing back out to sea.
"If the smell is problematic, I suppose people could try and bury them or maybe even use them in their garden. There's not much you can do about it, it's like mass standings of seaweed that can smell too, so it might take a few weeks but eventually they'll just degrade and go away."
He said Velella were at the mercy of the wind and thought there had probably been influxes of them for thousands of years.
Gordon said people may not have noticed them before because west coast beaches are not as well populated as east coast ones.
He said the first recorded mass stranding in New Zealand was in 2006 but reports had previously come from places across the world including California, British Columbia, Ireland and Scotland.
