Four people, including two children, from Waiau, North Canterbury have contracted norovirus in the last two days, Canterbury DHB says.

The patients were from three different families and were isolated in their own homes.

Civil Defence said most healthy people who contracted the highly contagious gastro bug usually got better in a few days.

However, norovirus was dangerous to frail elderly and people with weak immune systems.

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Civil Defence closed its welfare centre and communal kitchen in Waiau because some of the people who had been diagnosed with the bug had visited the centre recently.

Canterbury Medical Officer of Health Dr Alistair Humphrey said gastro bugs were often spread through the use of communal kitchens.

"The good news is that most households in Waiau now have power and running water so families can cater for themselves at home."

"Scrupulous handwashing" could prevent the spread of gastro, Humphrey said.

"I can't stress enough the importance of cleaning your hands thoroughly before preparing and eating food and after you've been to the toilet. If you don't have running water, use hand sanitiser to clean your hands."

No cases of gastro had been reported in Kaikoura, Humphrey said.

"This is a credit to everyone who is managing to keep up their hand hygiene despite the trying conditions and lack of water."

Food safety was also imperative to prevent gastro infections, he said.

"Any food that defrosted while the power was out should now be disposed of. All hot food should be served piping hot, and all fresh meat, seafood and poultry should be kept chilled in the fridge.

"Anyone with any gastro symptoms should definitely keep out of the kitchen."

Residents still needed to boil water before they had a drink or used it for cooking or cleaning their teeth.

Those who were unable to boil water should add quarter of a teaspoon of household bleach to two litres of water and leave to stand for two hours before using, Humphrey said.

People with gastro symptoms, including vomiting and diarrhoea, shyould stay home and phone their GP, Humphrey said.

A Kaikoura Health nurse was available 24/7 on 03 319 3500.