People are being warned not to eat shellfish caught in the Broken Bay area at the mouth of the Hawkesbury River, north of Sydney, due to possible health risks.

The NSW Department of Primary Industries (DPI) is advising people not to consume shellfish such as oysters, mussels, scallops, cockles and clams - or the guts of crabs and lobsters - caught around Broken Bay.

The warning comes after an outbreak of toxic algae bloom, which can cause Paralytic Shellfish Poisoning.

Recreationally harvested shellfish should not be eaten because algal toxins were harmful to people, a DPI statement said on Tuesday.

Deputy Director General for Biosecurity and Food Safety John Tracey said the affected area includes the waters of Broken Bay upstream to Cowan Creek, the coastal zone between Little Beach (Bouddi) and Bangalley Headland, and Pittwater.

"Paralytic shellfish toxins are produced by certain toxic algae species and shellfish such as oysters, mussels, scallops, cockles and clams should not be taken or consumed from this area," Dr Tracey said.

"With Christmas just around the corner, it's important that communities in the Broken Bay area take extra precautions when collecting and consuming seafood."

Cooking the product does not remove the risks.

Symptoms usually appear within 10 minutes to three hours of eating and may include:

* numbness and a tingling (prickly feeling) around the mouth, face, and extremities

* difficulty swallowing or breathing, dizziness and headache

* nausea, vomiting and diarrhoea

* paralysis and respiratory failure and in severe cases, death.

"Anyone experiencing these symptoms after eating seafood from or near the affected area should seek immediate medical attention," Dr Tracey said.