Far North District Council is warning residents to be weary of the water they buy to replenish rainwater tanks as they believe their supplies may be being siphoned and sold untreated.

The council said today there was "strong evidence" water had been taken from the council's Okaihau and Rawene supplies.

They believe the thefts were probably made using a fire hydrant.

"This is potentially a serious public health issue," the council's infrastructure and assets manager Jacqui Robinson said.

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"Our fear is that those taking water from council may also be taking water from streams and selling it as treated water.

"Residents should ensure that the bulk water carrier they are using is registered with the Ministry of Health.

"Drinking Water New Zealand lists all registered water carriers on its website and we will also have that link on the [council] website."

Water carriers should be able to supply delivery dockets stating the quality of the water and where it came from.

They should also provide a current water carrier compliance certificate.

Customers who are unsure whether a water carrier is legitimate can contact a drinking water assessor from the DHB's Public Health Unit on 09 430 4100.