About 13 per cent of the country's 5-year-olds have such poor oral health it affects their eating, sleeping and school work, an oral health expert says.
Professor Bernadette Drummond of Otago University's dentistry faculty, will be speaking on the subject at the Paediatric Society of New Zealand 65th Annual Scientific Meeting in Dunedin on Friday.
"Over 40 per cent of New Zealand 5-year-olds still get dental decay and of those, approximately about a third of those will have very significant problems that actually prevent them from eating properly, from sleeping, they may have pain that affects their schooling - so that's very significant."
Of children age up to 12, about 40 per cent had problems with decay, she said. Her figures came from 2011 Ministry of Health data.
Serious dental health problems could also affect children's behaviour, Professor Drummond said. "People often don't realise why they're grumpy children."
As part of Professor Drummond's research, families were interviewed before and after their children's teeth had been fixed, and found significant differences with 48 hours of treatment.
"Children go back to a normal diet, they sleep through the night - parents aren't taking time off work looking after their child with toothache, or losing sleep because they are waking in the night."
Children who needed major dental work when they were 4 were four times more likely to suffer tooth decay when they were teenagers, Professor Drummond said.
"We were finding things like they had poor gum health ... they had a tendency to be in the overweight or obese category, so obviously diet is playing a huge role here."
They also had a poor self image, she said.
"Having significant problems actually does carry through (to later years)."
The most critical factor in having poor dental health was diet, Professor Drummond said.
"The key message about diet is the frequency of eating carbohydrates - so we can find links (to tooth decay) to the increased use of sweet drinks, and that's not just the fizzy drinks ... it's juices, it's cordial.
"Those drinks are all very acidic, so we are seeing a huge increase in erosion where actually the whole surface of teeth are eroded."
Daily teeth brushing with fluoride toothpaste was also a factor in keeping teeth healthy, Professor Drummond said.
"We know that the decay is about a third more severe if children live in a low-fluoride versus a fluoride area."
Professor Drummond said that was graphically illustrated in differences between Dunedin, which had fluoride in the water, and Mosgiel which did does not.
"We see a big difference in tooth decay between the two areas," she said.
Many children with tooth decay came from low socio-economic backgrounds in which parents who could not afford dental treatment passed bacteria from their own teeth to their children's.
"If the mothers can't afford to have their teeth fixed, then they are passing those bugs onto their children all the time," Professor Drummond said.
Some families on the breadline simply could not afford toothpaste and toothbrushes as well as food, she said.
"I think we have to be realistic, there are people in this country that simply do not have that money."
Low cost options to help with oral health:
* use mouth washes;
* chew non-sugar gum that contains xylitol;
* brush with fluoride toothpaste; and
* steer clear of fizzy drinks, fruit juice and cordials.