There is an unusually high number of toxic algal blooms around the country for this time of year, the Ministry for Primary Industries says.
There are currently four warnings in place against shellfish collection due to algae - Bay of Islands, Pelorus Sound, the West Coast of the North Island and Akaroa Harbour in Christchurch.
The latter two were added in the last few days despite it being the coldest time of year.
Specialist advisor Brian Roughan said it was unclear why the blooms had fired up now.
"It's odd ... over this summer the Tasman Sea was extremely warm and it could be symptomatic of that."
The blooms are all the same type of algae, Alexandrium cattenella.
In May, Alexandrium cattenella was at its peak in the Pelorus Sound in Marlborough, and resulted in the closing down of 150 mussel farms.
Mr Roughan said the bloom in Pelorus was now away from the farms and only remained in Nydia Bay where it had started from.
However, farmers were still waiting for the toxins to clear from the mussels, which could take a few weeks.
He said the bloom on the West Coast happened late last year, but it was bigger this time around.
"It's extending down to Foxton Beach and right up to Raglan. We're monitoring on either side of it to see if we need to increase the warnings."
Warning signs had been put up on beaches and people were being urged to keep an eye on the MPI website or subscribe to biotoxin alerts, Mr Roughan said.
"People really need to heed the warnings, the levels can be mild - from tingling of the lips and numbness - right through to symptoms of nausea, vomiting, diarrhoea and in severe cases when there's high levels of the toxin, it can lead to death which has happened overseas."