Memo mums and dads: It's time to get the kids out playing with hammers and nails, gardening tools, getting stuck in places hard to get out of and play fighting.

Hair-raising? Yes, but rough-and-tumble backyard activities like these - or real play - are being hailed as weapons in the fight against the growing child obesity crisis.

Dr Scott Duncan, Associate Professor in physical activity and health at the Auckland University of Technology, says they are not only wonderful and wild things for kids to do, but are important learning experiences which can help enhance a child's wellbeing.

The 'games' are among a range of suggestions in the Ministry of Health's Sit Less, Move More, Sleep Well activity guidelines for under five-year-olds.

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Based on key international evidence, the new guidelines were produced by the ministry and an advisory group of experts on physical activity and child health and well-being.

Promoting regular active play, limited sitting and good-quality sleep as ways to combat obesity, the guidelines say children who play regularly tend to have healthier weights, develop better movement skills and reduce the risk of obesity than those who are physically inactive.

Duncan, who chaired the advisory group, says children need challenging and risky play - under parental guidance only if absolutely necessary - rather than being stuck inside for long periods on tablets, computers and game consoles.

He says real play also helps children feel independent, develops leadership and gives them confidence and resilience.

Duncan says there is growing concern an increasingly risk averse society is contributing to a generation of "bubble-wrapped" children with limited opportunities to learn to manage risks and overcome challenges.

Activities such as playing in the rain, splashing in mud, play fighting, climbing trees, playing with hammers and nails, gardening tools and getting stuck in places hard to get out of are all great ways for kids to learn how to manage and overcome risk he believes.

"In the long run we will do them a disservice if we don't allow them play freedom because when they are older they will have no appreciation of risk," he says. "We are not talking about hazardous play, we don't want that, but there is a place for risk."

The guidelines are part of the government's Childhood Obesity Plan. The latest New Zealand Health Survey shows 11 per cent or one in nine Kiwi children aged 2-14 are obese while seven per cent aged 2-4 are obese. A further 21 per cent are considered overweight but not obese.

Duncan says State of Play, a 2015 AUT report he co-authored, found most New Zealand parents recognise the benefits of real play - although this did not necessarily translate into actual play practices.

"Our research showed a majority of children do not often participate in a wide range of real play activities," he says. "In fact, many do not engage in real play at all."

Duncan said the report showed nearly 83 per cent of children spent more than the recommended two hours per day in front of a screen during the week (88 per cent on weekends) while 58 per cent did so for more than four hours every weekday (62 per cent on weekends).

"It is not that screens are inherently bad," he says. "They should not be banned or frowned upon, the issue is the amount of time children are spending on them."

Duncan says it is understandable parents worry about their children being exposed to risk.

"Most parents can recall weekends spent riding their bike to their friend's house, exploring the local bush or walking to the nearest shopping malls but today less than half kids aged 8-12 are allowed to travel alone in their neighbourhoods.

"Parents identify the risk of road accidents or encounters with ill-intentioned adults as reasons not to let them," he says, "but the perception of risk is much greater than the actual risk and when perceived risks become elevated well beyond reality, opportunities that would otherwise be beneficial to children's development disappear."

Duncan says in the last 30 years children have shifted more to structured, supervised and/or indoor activities: "Paradoxically a lowering of opportunity for kids to indulge in risky, independent play may generate much greater risks in terms of impaired physical and emotional development.

"But it is hard for parents; it is harder to make a buck, to buy a house, everyone is working harder and parents are under pressure."

The Sit Less, Move More, Sleep Well guidelines urge parents and caregivers to promote fun activities to support physical, social, emotional and spiritual growth - at least three hours a day for toddlers and pre-schoolers.