A new study shows two in five children living in Auckland and Northland have one or more decayed, missing or filled tooth by the age of five.
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The University of Auckland study in the the New Zealand Medical Journal said the rate was even worse for Māori and Pacific children - with 59 percent and 65 percent respectively - and children from very deprived neighbourhoods with 60 percent.
This compares to rates of 25 percent in England, 35 percent in Wales, and 34 percent in Queensland, Australia in a similar age group.
The study also showed children who had been hospitalised for an injury were more likely to have rotten teeth when they started school.
Researchers said the higher rates in Northland could be because of the lack of fluoridated water in the region.
Researchers said their findings showed the "urgent need for society-wide actions to improve oral health in young children and tackle inequalities".
"Including water fluoridation, measures such as a tax and health warnings to reduce consumption of sugary drinks, and oral health promotion that is meaningful in communities where children currently experience poorer oral health," researchers said.
The study was based on records from more than 25,000 children from 2014 and 2015.
There has been a call recently for reform the sector, with reports of people unable to afford treatment ripping out their own teeth, and Health Minister David Clark saying people were dealing with "Third World health conditions" as a result of bad oral health and inability to access treatment.