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 so average number of fillings and number of children with with decay for five year olds from 2001 to 2019 (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.
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.
2005: The Southland Study
1950s: The Hastings Experiment