Chances of a gastro bug affecting Whangarei residents similar to the one that made hundreds sick from the water supply in Hawke's Bay are "very slim".

Whangarei District Council water services manager Andrew Venmore said different situations existed in Whangarei and Hawke's Bay as far as the source and treatment of reticulated and bore water were concerned.

Between 1000 and 2000 people have been affected by the bug in the town's water supply, which the Hawke's Bay District Health Board says was almost certainly campylobacter.

Twenty people are in hospital, including two who are critically ill, and all Havelock North schools have been closed as the community fights the spread of the vicious stomach bug.

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The town's supply has now been chlorinated to kill the bug, and Hastings District Council has brought in tankers of uncontaminated water so residents can fill containers from the street.

Mr Venmore said unlike Whangarei, Hastings and Havelock North got their water from a series of groundwater bores which were not treated or chlorinated because they were considered secure.

Whangarei District Council treated and chlorinated all its reticulated and bore water.

Mr Venmore said the water was also UV disinfected to ensure it was safe.

"We also test at test points across the district after it has left our treatment plants.

"The chances [of a gastro bug in Whangarei] are very slim. The type of bug Hawke's Bay residents have been getting is killed by chlorine," Mr Venmore said.