People collecting shellfish in Hawke's Bay need to make sure they are choosing clean spots, health officials warn.

In light of the paratyphoid outbreak last month, which has been linked to mussels collected from Ahuriri and consumed at two tangi in August and September, health officials are advising people to collect shellfish off-shore.

A number of cases have required hospital care at Hawke's Bay Hospital and another has needed treatment in Auckland.

Hawke's Bay District Health Board (HBDHB) has signs in Ahuriri warning people that the shellfish is unsafe to eat, however Hawke's Bay Today has received reports of people still collecting shellfish.

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HBDHB health protection officer Cameron Ormsby said as tempting as it might be, collecting shellfish from anywhere in Ahuriri could be dangerous.

This is because shellfish are filter feeders and get their sustenance from straining food particles from water meaning that they take in a high volume of water and become concentrated with contamination.

Shellfish from Ahuriri are at a high risk of contamination due to pollutants from being close to the city, overflow, as well as boats nearby, which (even though it is illegal) may put their sewerage into the water.

This is why mussells are usually farmed off shore in reasonably deep water with good water flow, he said.

"Personally, when I collect mussells I make sure I am 5km off shore and about 15-20m deep, and I don't do it after recent rainfall."

HBDHB medical officer of health Oz Mansoor said shellfish become contaminated even in places where people can swim because while the bacteria in the water is diluted it concentrates when going through the shellfishes filter feeder system.

"They are natural purifiers. I would never get shellfish from [Ahuriri] because there are too many contamination risks."

Samples collected at the Napier Marina on September 23 found contamination of E. coli and norovirus but no paratyphoid.

This showed that the rare paratyphoid was probably a one-off event but because other bacteria were commonly found in shellfish they should not be consumed, he said.

While the source was unlikely to ever be confirmed, the paratyphoid contamination probably came from an infected person who dumped their sewerage into the water, Mr Mansoor said.

Anyone who might have been contaminated is advised to seek medical advice because the illness can present itself without symptoms, he said.

Paratyphoid generally occurs within 10 days of consuming contaminated food or water but symptoms may take as long as four weeks to develop.

Most people with paratyphoid will have a fever, chills, headache, possibly a rash and may also get severe vomiting and diarrhoea.

Some cases in the recent outbreak have been linked to mussels consumed at a tangi at Tangoio Marae around September 11 and another held at Flaxmere's Te Aranga Marae between August 28 and 30.

Ministry for Primary Industries advice to lower risk of illness from contamination

* Avoid collecting shellfish from areas where pipes and culverts run down to the beach.

* Avoid collecting from areas where sewage or storm water is discharged, or areas with houses nearby.

* After heavy rain, don't collect near rivers or estuaries until the water has run clear for several days.

* Don't collect where there are farm animals grazing nearby.

* Don't collect from areas showing signs of industrial pollution.

* Avoid collecting near wharves or marinas where boats may have discharged sewage or chemicals such as anti-fouling paint or diesel.