Babies born before or during the gastro outbreak could have been protected from illness by breastfeeding.

New Zealand Breastfeeding Alliance executive officer Julie Stufken said breastfeeding was one of the best ways to prevent illness in young children, and it became even more critical in an emergency.

She said young children were the most vulnerable to infection and illness, because they had undeveloped immunity and dehydration concerns.

"The safest and cleanest food for an infant is human milk due to the specific antibodies and nutrients."

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The bacterial infection, campylobacter, could not be transmitted through breastmilk, so during a water-borne illness it was the best way to feed a baby, she said.

"Breastmilk is readily available at the right temperature and is protective against diseases, particularly diarrhoea and respiratory infections."

She acknowledged that for some women breastfeeding was not possible and said that extra precautions should be taken to reduce the chance of infection.

"It is important the infant formula is prepared correctly using boiled water, and sterilising bottles and teats."

Health professionals are also reminding women to take extra precautions if they are on the contraceptive pill and have the gastro bug.

Jeff Whittaker, from Unichem Whittaker's Pharmacy, said vomiting and diarrhoea, the two main symptoms from the water-borne illness, would reduce the effectiveness of a contraceptive pill.

"It was important to count each sick day as a day missed," he said.

Mr Whittaker said that if a woman had vomited within two hours of taking the pill she would have to take it again, because the pill would not have been absorbed in to her body.

"Women should look at other forms of contraception, as the pill would not work if you continued to be sick for 24 hours."

The seven-day rule should also be applied to those who had been unwell, before protection against pregnancy was guaranteed again.

The pharmacy had been giving out flyers and pamphlets to women. These were supplied by the Hawke's Bay District Health Board.

The DHB also had information on its website under the "frequently asked questions" tab which explained the reduced effectiveness of the pill.