Wellington Water has admitted most parts of the region have not had fluoride in the drinking water for up to a year.
The company revealed on Wednesday fluoridation hadn't happened for a month - but has since admitted it's been much longer.
An inquiry's been launched to find out what went wrong.
A water treatment plant in Upper Hutt hasn't been fluoridating drinking water since May, and one in Seaview stopped in November.
But just two days ago Wellington Water told residents it'd only been one month since fluoridation stopped.
"The chief executive has come and said I've given you the wrong information, he obviously felt terrible about that," Wellington Water Board chairwoman Lynda Carroll says.
The company doesn't know exactly who is at fault.
"Has someone at Wellington Water lied about the dates when the fluoride stopped?" Newshub asked.
"I don't know - that's why we are doing the independent inquiry," Carroll replied.
Dentists are shocked as fluoride helps prevent rotten teeth.
"The NZ Dental Association is absolutely appalled at the situation," NZ Dental Association Rob Beaglehole says.
"It's astonishing that Wellington Water haven't fronted residents and told them they weren't fluoridating water," Wellington City Councillor Fleur Fitzsimons says. "It's deeply concerning and a major public policy failure."
"We should have known that was the case so that we can do something about it," one resident says.
"It's an important health concern for some people - a lot of us rely on it for our children and grandchildren so they've ripped us off," another resident says.
The Capital's water treatment plants are over 30 years old and some have been failing to correctly dose fluoride for the past four years.
The Dental Association says there's been an increase in Wellington kids needing rotting teeth removed in the past year.
And believes the lack of fluoridation's partly to blame.
"We're very concerned about this. We know that children living in non-fluorodated areas have 40 percent higher tooth decay rate than those living in fluoridated areas," Beaglehole says.
Only about 2.3 million Kiwis have fluoridated drinking water, but that's set to increase under new legislation which gives decision making power to the Director-General of Health - who can force councils to put fluoride in their water.
And after this incident, there are calls for a review of those already fluoridating to make sure it's being done properly.
"If this is happening in Wellington maybe it's also happening in other regions," Beaglehole says.
The NZ Dental Association wants fluoride in all drinking water to reduce tooth decay, especially in children.