Far North residents buying bulk water are urged to check the supplier is an accredited contractor and the water comes from a treated source.
The warning comes after meter readings at Rawene and Okaihau indicated public supplies had been tapped in bulk volumes before last weekend's rain.
About 30,000 litres were taken from fire hydrants at Okaihau and 60,000 litres at Rawene. Hydrants are not locked as firefighters need to get quick access to them.
"We have no direct evidence of theft, but have had three instances of abnormal water usage at odd times consistent with a small tanker filling," a Far North District Council spokesman said.
"We can only ask that people who see trucks taking water from hydrants to call us and provide as many details as possible."
Jacqui Robson, Far North infrastructure and assets general manager, said water from an unlicensed contractor or untreated source was a potentially serious health issue.
Rather than the criminal aspect, the council's main concern was those tankers could supply water taken from untreated sources like streams or bores that did not meet Ministry of Health safe drinking water requirements
Licensed carriers can take bulk water from authorised supply points with security locks, which contractors are able to access.
Customers should ask to see paperwork before accepting a water delivery, Ms Robson said.
Carriers should have a current annual registration with the Ministry of Health and should be able to give customers delivery dockets stating the quality of the water and where it came from.
Kaipara manager Peter Marshall said the council was concerned people might take drinking water from untreated sources.
The council introduced restrictions at Dargaville and Baylys Beach in mid-January, and is keeping a close on the situation at Maungaturoto.
Kaipara has a cost-only agreement to take from the Whangarei public supply to get bulk water to Mangawhai residents.
In the driest district, the Far North council is reiterating that all customers connected to council water supplies, or with rainwater tanks or bores, need to conserve water to delay the introduction of tougher water restrictions.
While the weekend's rain replenished tanks, people are advised to check levels regularly and order water earlier rather than later as they could face waits of up to 10 days for delivery.
A delivery of 10,000 litres of water costs around $200 close to town and $500 for rural and coastal areas where cartage pumps up the price.
The Northland Regional Council has not declared a drought, which it could possibly do in collaboration with Niwa and the Ministry for Primary Industries should the dry weather and lack of ground water continue into the next few weeks.
Meanwhile, anyone with concerns about the legitimacy of a delivery can contact the Health Protection Officer for a water test, through the district health board's main switchboard, 09 430 4100.
Customers can also check a carrier's MoH registration on the Drinking Water NZ website: http://www.drinkingwater.org.nz/carriers/carriersfornzregion.asp?NZRegion=NZNZ02