An independent inquiry has been launched after Wellington Water was found to have overseen low and inconsistent levels of fluoridation in the region's water supply for four years. 

Fresh revelations have also emerged tonight that Upper Hutt, Porirua and Wellington City have not been supplied with fluoridated water since last year without residents being told. 

Wellington Water previously reported fluoride had been turned off last month, which was incorrect. 

A recent review found the fluoride dosing facilities at water treatment plants in the region have been operating inconsistently because the machinery is getting old. 

As the machinery could not be relied on, operators have been conservative with the amount of fluoride being put into the supply, Wellington Water documents said. 

This is because there are health risks posed by high levels of fluoride, but it has also resulted in periods where doses have fallen below the effective range. 

This "monitoring and nursing" of the situation recently became "too onerous", the documents said. It was only at this point a review was commissioned. 
 
Tonight the board of Wellington Water announced an independent inquiry into the events that resulted in two of Wellington's water treatment plants ceasing fluoridation of drinking water. 

The board has clarified fluoridation was stopped at the Te Marua water treatment plant in May 2021 and in November 2021 at the Gear Island plant. 

They supply Upper Hutt, Porirua and Wellington City. 

Wellington Water board chairwoman Lynda Carroll said this raised serious questions for the board. 

"To discover that there have been issues in the plants for such a period of time without this being brought to our attention, and then to find out that there has been a failure to release accurate timelines and information that ought to give our stakeholders assurance is disappointing to say the least." 

Carroll said the inquiry would look into Wellington Water's management of the plants, how the situation occurred and what the board and management should have known. 

The inquiry would also assess what, if any, issues may have blocked the board from being aware of the problem earlier. 

The terms of reference will be made public, Carroll said. 

"This is something we are digging into straight away. Standing up the inquiry is a priority for us, but this won't stop us from working to get fluoride back in Wellington's water supply as soon as possible." 

Doug Martin of Martin Jenkins has been appointed to lead the inquiry. 

Meanwhile, work was under way at Wellington Water to establish timelines, gather records and develop reporting on fluoridation levels at all facilities over the past four years, Carroll said. 

The Wellington Water Committee has been informed of the independent inquiry, and supported the board's moves, she said.