While most people in urban Auckland get fluoride in their water, many in outlying rural areas don't.
The chemical isn't added at water treatment plants in Bombay, Bucklands Beach, Clarks Beach and Waiau Beach, Algie Beach and Snells Beach, Glenbrook Beach, Patumahoe, Muriwai, Warkworth, Waiuku and Wellsford.
And not all of urban Auckland's water is fluoridated - plants at Onehunga and Huia Village don't use it.
Auckland Dental Association fluoride spokeswoman Dr Heather Keall said it was difficult to tell differences in oral health among people who lived in fluoride-free pockets of the city.
But a lack of fluoride was clearly noticeable in patients from Northland or rural areas around Auckland.
