It is feared there could be a jump in the number of children who drown this summer because of a deadly combination of kids missing swimming lessons and too many parents being unaware of water safety measures.

A survey of 2036 Australian parents by the Royal Children's Hospital in Melbourne found one in six children aged between six months and 17 years old has never had a swimming lesson.

It found 47 per cent of Australian children missed out on classes or other swimming opportunities because of the pandemic, with youngsters in Victoria and NSW most affected.

About one in three primary school-aged children could not swim or only had beginner skills, while one in 10 teenagers was in the same position.

Almost half of all parents surveyed incorrectly believed it would take more than 20 seconds for a child to drown, and 49 per cent did not know that young children need to be within arm's reach in the water at all times.

About one-quarter thought it was OK for a child to swim alone if they were between flags at a patrolled beach, while 29 per cent thought it was fine to leave a toddler in the bath if an older child was watching over them.

Drowning was the leading cause of death for children aged 15 or under in 2021, and it is feared many families may underestimate the risks posed in holiday areas over the coming months.

"The combination of reduced water-safety skills among children and low levels of parent knowledge about water-safe practices means that Australian children are at increased risk of drowning and near-drowning accidents this summer," paediatrician Dr Anthea Rhodes said in the report.

"Public health messaging and education for parents and carers about safe supervision and other water-safe practices is urgently needed."

The report found children were more likely to have swimming lessons with a qualified instructor if they lived in higher-income households or their parents were born overseas.

It also found mothers were more familiar with water safety measures than fathers, parents from regional areas knew more than those living in urban areas, and parents born overseas were less aware of safety risks compared with those born in Australia.