Dentists are pleading with parents to cut the amount of sugar in their children's lunch boxes, as figures show NSW is in the grips of a child tooth decay crisis.

More than 100 children are having multiple rotting teeth extracted, filled and capped under general anaesthetic each week, the latest NSW hospitalisation data shows.

Tooth decay and other preventable dental problems landed 16,700 NSW adults and children in hospital in 2015-16.

"It's absolutely a crisis when you also consider the amount of suffering, the amount of time parents have to take off work, and the cost to the government and to the parents," said paediatric dentist Dr Sarah Raphael from the Australian Dental Association NSW.

Dentists say some of the biggest culprits are highly processed, sugary foods and drinks, and as families gear up for the new school year, they are urging them to cut the empty calories.

A Fairfax Media analysis found that in extreme cases a school lunch box consisting of five basic items can easily contain more than 160 grams of sugar, or 40 teaspoons.

Australian adults and children should consume no more than 51 grams in free sugars a day, according to World Health Organisation guidelines, and halving this amount can lead to extra health benefits.

The term "free sugars" extends the definition of added sugars to include sugars naturally present in honey, fruit juice and fruit juice concentrates.

Dr Raphael said apart from obesity and weight-related health problems such as type 2 diabetes, consuming too much sugar was linked to tooth decay because the bacteria that breaks down food and drinks in the mouth produced acids that erode the tooth enamel.

She said parents should be extra wary of sweet sticky food that can get stuck in the grooves of the teeth and always provide water so that sugars and acids can be washed away.

"We're seeing kids from 18 months with decay and the peak age range for paediatric dentists treating children under general anaesthetic because of decay would be three to six years of age," she said.

"Decay affects the way children smile, socialise, eat and even speak properly, and these kids are going to form speech patterns that are more awkward."

About one-third of NSW children aged five to 10 have decay in their baby teeth and one-fifth of children aged six to 14 have decay in their permanent teeth, the National Child Oral Health Study 2012-14 found.

More than 5500 children aged up to 14 were hospitalised for tooth decay-related operations in 2015-16, up 48 per cent on the figure a decade and a half ago, NSW Health statistics reveal.