Sophie Beaumont, a dentist at Victoria's public dental hospital, said some children were presenting with blackened teeth and pus-filled gums, suspected to be caused by soft drinks and high sugar diets.

Some parents delayed going to a dentist until their child was in so much pain that the child's cries were keeping the family awake at night, she said. And some children's mouths were so bad they had to have all 20 of their baby teeth removed.

"It is very sad because you can imagine the impact on that child when they have to go to school with no teeth and try to function without them. It is quite upsetting," she said.

"It is not uncommon to be taking out 12 or 14 baby teeth in one go. We would do that quite frequently."

More than 1000 children throughout the state were put under general anaesthetic at the Royal Dental Hospital of Melbourne last year, mostly to get multiple decayed teeth removed. Of these, 178 were three or under.

Dr Beaumont said the extractions often had lifelong consequences for the child – not least a permanent fear of dentists – because of problems with crowding and alignment that can occur when adult teeth emerge. This can only be fixed with expensive orthodontic treatment, which is not always available through the public dental system.

Frequent fizzy drinks and other drinks high in sugar such as cordials or fruit juices were the main culprit for large-scale decay, she said.

Meanwhile, the Australian Dental Association's David Manton said as many as 10 per cent of children who were being put under a general anesthetic to have rotten teeth taken out were returning a later date to have more removed because their diet had not improved.

Professor Manton, the deputy chair of the dental association's oral health committee, said non-English speaking or Indigenous children and those from low socioeconomic backgrounds were most at risk of large-scale tooth decay.

Young children are typically put under general anaesthetic if multiple teeth have to extracted, but if just one tooth needs to be removed it can be taken out in a dentist's chair.

A total of 164,785 children received dental care from the Dental Health Services Victoria in the past financial year, many of them for tooth decay. The health agency also runs awareness campaigns on caring for teeth.

Professor Manton said the Australian average for four-year-olds was to have one tooth with decay, with most of those remaining untreated.

He said a recent survey of more than 600 children from inner Melbourne preschools found that just over one-third of the children had evidence of decay.