More than 30,000 Australian children may now be severely obese, a national study into childhood health has found.

Researchers said their findings suggested they had underestimated the problem, and its extent may be even greater than figures show.

"The numbers on how many Australian children have severe obesity have never been crunched until now and our findings suggest we have underestimated the issue," Associate Professor Sarah Garnett from Sydney's Westmead Children's Hospital said.

The study, which used four national surveys looking at children aged seven to 15 from 1985 to 2012, showed the rate had significantly increased over time.

The biggest rise was in 2007, when the number of obese children with severe obesity jumped by 50 per cent — from 20 per cent in 1995 to 30 per cent.

Associate Professor Garnett said there were concerns the extent of the problem had been underestimated because some children were opting out of the research.

"We are a little concerned that maybe some of the heavier children aren't opting in to be involved in the study," she said.

"Certainly when we look at the 2012 survey, I think about 20 per cent of the households that were involved, the children actually opted not to be weighed and measured."

Unlike overweight and obese children, children with severe obesity require specialist care and have the highest risk of additional health problems, Associate Professor Garnett said.

"Failure to treat these children will have huge implications for the individual and our healthcare system in the years to come," she said.

"Currently there are not enough paediatric obesity services to look after these children."