Investigations are ongoing into the cause of the bacteria outbreak which closed Pandora Pond twice last week, and a possible related illness.

A swimming warning remains in place at the Napier spot.

The initial warning was put in place on Wednesday after a high reading of 475 Enterococci per 100ml, lifted on Thursday after two clear samples, and reinstated on Friday after a reading of 302 Enterococci bacteria per 100ml.

Readings of this level equate to about one in 15 pond swimmers getting a stomach illness, and about one in 30 getting a respiratory infection.

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Today Hawke's Bay District Health Board medical officer of health Dr Nicholas Jones said the DHB had been "fielding calls" and was investigating one possible case linked to Pandora Pond activity from early last week - before results were returned and swimming warning issued.

This case had been notified by a GP. The HBDHB had issued a medical advisory notice to general practices to assist with public health surveillance of any pond-related illness.

There had been concern with swimming allowed one day last week, between the warning being lifted Wednesday night, and reinstated on Friday.

However Dr Jones said samples between these times were compliant within the safe swimming guidelines - 280 Enterococci bacteria per 100ml.

"This means that swimmers using the pond prior to the reinstatement of the warning were not at risk.

"Over the weekend results have continued to fluctuate and a swimming warning remains in place for this reason. We ask people to see their doctor if they are feeling unwell following swimming in Pandora Pond last week and to advise their doctor what day they had swam in the pond."

Taradale Intermediate School students were to go sailing on the pond while the warning was lifted, but Principal Rex Wilson said in hindsight they were glad they had taken the precaution not to.

He said they had decided to call off the activity as "we couldn't be sure the conditions were safe for the kids".

Even though the students would be sailing in Hawkes Bay Optimist Yacht Trust vessels, he said the school could not take the risk of one child falling into the water.

"Safety is the priority with all of these things. While it is disappointing...ultimately their health and wellbeing is the priority."

Another lesson had been scheduled for today. Although making these up to the students would be a "challenge", Mr Wilson said they trusted the councils, and HBDHB would make the right call.

Investigations are continuing into the cause of the contamination.

HBRC Group Manager Resource Management Iain Maxwell said staff had been taking daily, five litre samples since the initial contamination, which continued over the weekend at five locations around Pandora Pond and Ahuriri Estuary.

Daily samples would be taken until further notice. Mr Maxwell said small amounts of each sample were sent for testing, with results received within 24 to 48 hours.

- For more information on where to go for medical care go to: ourhealthhb.nz/choose-well/where-should-i-go/, or call Healthline: 0800 611 116 for free, for advice from a trained registered nurse.