A chain of conditions has led to warnings for people not to collect shellfish anywhere inside the Bay of Islands due to a potentially deadly toxin.

Higher than usual levels of paralytic shellfish toxins (PST) in shellfish has seen the Ministry for Primary Industries issue a health warning for the area from the outer heads between Cape Wiwiki on the north to Cape Brett on the south. The warning includes all inlets and estuaries. Similar warnings are in place in Hawke's Bay and parts of Marlborough Sounds.

But the ban is not expected to affect the Bay's multi-million dollar oyster industry.

The latest weekly sample testing has shown PST at levels above the safe limit of 0.8 mg/kg set by MPI. The toxins develop in filter feeding shellfish that have fed on blooms of microscopic dinoflagellate algae. The chain reaction can lead to paralytic shellfish poisoning (PSP) in people.

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The shellfish look and taste normal, despite their poisonous cargo. Mussels, oysters, tuatua, pipi, toheroa, cockles, scallops, cat's eyes, kina (sea urchin) and all other bivalve shellfish should not be eaten. Pāua, crab and crayfish can be eaten if the gut has been completely removed before cooking.

Bay of Islands oyster farming spokesman Ben Warren said that industry was unlikely to be affected by current elevated toxicity levels.

''It's pretty routine. We monitor for this all the time. Part of what we do as an industry is contribute to the national testing programme.''

Shellfish farmers tested regularly for a range of conditions and stopped harvesting long before toxin levels or other factors affected the crop, Warren said.

''You can detect it in such minute amounts.''

As for public health warnings about collecting shellfish: ''My message is, if there's a warning posted, don't eat the shellfish.''

Anyone who becomes ill after eating shellfish from where a warning is in place should seek medical attention immediately.

In Northland in 2003, a woman who ate shellfish from a Far North banned area was admitted to hospital with PSP, and recovered. In 2012, more than 20 people were poisoned in the Bay of Plenty shoreline, with 10 of them admitted to hospital.

PSP is responsible for many deaths overseas each year, with the American continent's coast being high risk.

Bans on taking shellfish from around New Zealand's coast occur fairly regularly. The last ban over the entire Bay of Islands was in February 2017. Last November the North Island's west coast from south Taranaki to the Far North was out of bounds for several weeks. The danger usually passes as the algal bloom disperses and shellfish excrete the toxins.