Auckland's water provider has quietly reduced its target for fluoride levels in the region's drinking water.
But it denies the move was clandestine.
Watercare has reduced its target fluoride levels from 0.85 parts per million to 0.7ppm.
The Ministry of Health recommends fluoride content for drinking water in the range of 0.7-1.0ppm and must not exceed 1.5ppm -- the acceptable maximum level.
A Watercare spokeswoman said the 0.15ppm drop was part of a continuing effort by the organisation to follow best practice standards.
She said this, "combined with recent international reports, resulted in an operational adjustment of the fluoride target".
In April last year, the US Department of Health and Human Services updated its Public Health Service recommendation for the optimal fluoride level in drinking water to prevent tooth decay to 0.7ppm.
Anti-fluoride lobby group Fluoride Free NZ questioned why Watercare had not notified the public about the drop, calling the move "clandestine" in a press release.
The group said the new level was quietly revealed in a Watercare report last month "without announcement or fanfare from Watercare or Auckland Council".
Fluoride Free NZ spokeswoman Mary Byrne questioned why the public had not been directly notified.
"They're giving it [fluoride] to people without their consent and without their knowledge, mostly. I mean look at this, they're lowering it and they're not bothering to tell anybody.
"They're not keeping people informed."
Ms Byrne said she was happy the target amount had been reduced, but wanted fluoride taken out of the water altogether.
"It's a step in the right direction. It's indicative of what's happening with fluoridation."
But a Watercare spokeswoman said the move was not deliberately secretive and the January report was publicly available.
The public had not been notified as the change was not significant, she said.
New Zealand Dental Association spokesman Dr Rob Beaglehole agreed, saying Watercare was simply updating best practice and it was not necessary to alert the public to the drop.
The Dental Association was happy with the council's move as 0.7ppm would still provide enough fluoridation to deliver dental benefits for people's teeth, especially as many people were now using toothpaste with added fluoride.
"It's still going to be effective," he said, adding that 0.7ppm was still within the 0.7ppm - 1.0ppm range the Ministry of Health recommended.
Dr Beaglehole said the association favoured fluoride in drinking water as it was the best way to deliver its benefits to the whole population.
Councils currently have the power to keep, add or remove fluoride from the water supply and the fluoride debate has been raging in New Zealand for years.
In June 2013 Hamilton removed fluoride from its water following public consultation, only to put it back in in May 2014 after a citizen-initiated referendum.
Last month the Whakatane District Council voted to stop adding fluoride to any of the district's public water supplies.
In 2014 a high-level panel found no adverse effects of fluoridation of public water supplies after a review of scientific evidence.