The Ministry of Health collect statistics from the NZ School Dental Service. Every year they provide the percentage of children with no dental decay and the average number of fillings for those that do have decay. They provide these figures for 5 year olds and Year 8 children for every year since 1990. There are about 45,000 children in each age group.
Below graphs show average number of fillings and number of children with with decay for five year olds from 2001 to 2020 (latest data). Over that time the gap has become non-existent because the rates of decay in fluoridated areas has not improved but rates in non-fluoridated areas has improved.
2016: Water fluoridation and ethnic inequities in dental profiles of New Zealand children aged 5 and 12-13 years
NZ study comparing children in fluoridated to nonfluoridated areas. They found there is no longer any difference in decay rates for non-Maori children (this includes Pacific Island children). Difference found for Maori children is likely to be the result of rural-urban divide.
2013: Fluoridation and dental caries severity in young children treated under general anaesthesia: an analysis of treatment records in a 10-year case series. Kamel, Thomson and Drummond, Community Dental Health March 2013.
2009: the Ministry of Health carried out an Oral Health Survey to look at dental health across the nation. The Ministry of Health cite results from this survey, despite the fact that it was a snapshot in time with a very small sample size. See more here.
2009: Enamel defects and dental caries in 9-year-old children living in fluoridated and non-fluoridated areas of Auckland
2008: Prevalence of enamel defects and dental caries among 9-year-old Auckland children
2005: The Southland Study
2004: The Wellington-Canterbury study
1998: The decline of caries in New Zealand over the past 40 years. Betty de Liefde. NZDJ 94: 109-113.
1950s: The Hastings Experiment