A 4-year-old girl who had all of her rotting baby teeth removed is one tens of thousands of Kiwi kids with poor dental health, a new report has found.

Dental caries, or tooth decay, was found to be most common chronic disease in children, and one of the leading causes of hospital admissions.

In the 2015/2016 year, 29,000 children under the age of 12 had to have one or more teeth removed as a result of severe cavities and infection, and 6600 of those ended up in hospital. Each operation to remove teeth cost about $4000.

"The cost of this in public healthcare and to taxpayers is huge," oral health researcher and co-author Prathibha Sural said.

The report, Too soon for the tooth fairy: the implications of child poverty for oral health, summarises knowledge about the prevalence of poor oral health among children in New Zealand.

Most of the cases were argued to be preventable and strongly associated with poverty.

Co-author and public health specialist Dr Rob Beaglehole said tooth decay was a "disease of poverty".

"Poorer members of society have significantly worse oral health than the wealthy."

Poor oral health had a knock-on effect to other areas of life.

"Persistent orthodontic problems, as well as social issues for children can occur as a result. They may require expensive orthodontic work on adult teeth, and their confidence suffers.

"Education so that families are more aware of proper nutrition and care for their children's teeth from before they erupt is imperative, but families must be resourced with the kind of incomes they need to sustain healthy lifestyles if any long-term change is to be seen."

CPAG health spokesman Professor Toni Ashton said despite oral health being a stated priority of governments over the past two decades, dental problems among New Zealand children, especially in low-income families, remained stubbornly high.

"Mere lip service has been paid to this problem for far too long. It is time for action."

The report suggests significantly boosting family incomes, a comprehensive, parent-targeted education programme, broader provision of fluoride in water, reducing the sugar content in food, and the implementation of a health levy on sugary drinks.

The University of Otago's Dr Jonathan Broadbent said child poverty and its detrimental impact on dental health care cast "a long shadow and affects oral health into adulthood".

A University of Otago's Dunedin Multidisciplinary Health and Development Study, which tracked the lives of more than 1000 babies born in Dunedin between April 1972 and March 1973, showed up to 18 years of age, more dental decay occurred among children from socio-economically disadvantaged families.

While those who needed dental care mostly got it due to New Zealand's universal dental health care for children and adolescents, this changed once access to state-funded dental care ended at age 18.

By the time they were 26, the average number of teeth with untreated decay among young people from socio-economically disadvantaged families was five times greater than it had been at age 18.

"Publicly funded dental care minimises inequality up to age 18, but then the bottom drops out," Dr Broadbent said.

By age 38, the average number of teeth lost was six times greater among those who had been born into disadvantaged families than for those born into well-off families.

Socio-economic differences in tooth decay rates didn't explain socio-economic inequality in dental health alone, because well-off people who ate too much sugar and had poor oral hygiene had a high rate of tooth decay too, Dr Broadbent said.

"But, if you have a high rate of decay as a child and are born into a family with low-income parents, this will affect your risk of having poor dental health right through your life, not just during childhood."

Another study found socio-economically disadvantaged parents were less likely to understand the dental problems caused by sugary foods or how to effectively care for teeth.

Their beliefs rubbed off on their children who were more likely to hold similar unfavourable beliefs through their teens and into their 20s.

They were less likely to regularly brush their teeth or go for regular dental checks.

Dr Broadbent suggested public health interventions targeted to reduce inequalities, like a sugary drink tax, and expanded publicly funded dental care, including preventive care for adults.

CALLER JENNY TOLD ANDREW DICKENS ABOUT WITNESSING THIS FIRST HAND