Jack MacMillan drowned in a metre of water in his  backyard, a short distance from his mum, who never let him or his sisters in the pool unsupervised. 

Two days before he was to start high school, he drifted quietly to the pebblecrete floor at the shallow end while swimming underwater laps – not splashing, not distressed, not fighting death.

The coroner would later look for a bump on his head or some outward sign of what had claimed his life, but there were none.

The problem had occurred inside Jack's body and was - as his family so fiercely want it known - the result of a common, ages-old game.

Twelve-year-old Jack, of Cordeaux Heights in the NSW Illawarra, had been swimming underwater laps before he died on January 29.

He became so focused on his personal target he ignored his body's urge to breathe and lost consciousness.

When he did take his next breath, his lungs filled with water, not air, and he quickly drowned.

Jack's aunt, Sharon Washbourne, is leading a campaign to raise awareness of the set of circumstances leading to his death, known as "shallow water blackout" or "hypoxic blackout".

Mrs Washbourne is concerned that the threat is going unrecognised, partly because deaths are sometimes attributed to more general causes such as drowning.

Mrs Washbourne is working with Royal Life Saving NSW and has written to Sports Minister Kate Lundy in the hope of making shallow water blackout as well known as "Slip, Slop, Slap".

"This is something that every child is doing - adults are doing it," Mrs Washbourne said.

"Jack nor any of us knew the consequences of holding your breath too many times repetitively.

"You literally have seconds to pull someone up before brain damage or death occurs."

Exhaustion is a key contributor to blacking out underwater, as is exertion, which can cause oxygen levels to deplete more quickly than in a body at rest.

Family members believe Jack could have remained conscious if the periods between his underwater laps had been longer, allowing his oxygen and carbon dioxide levels to return to normal.

His mother, Michelle MacMillan, said Jack was a capable swimmer who loved being in the water.

"When we were looking to buy a house the first question was always: 'does it have a pool?'," said Mrs MacMillan, a child protection worker.

"Most days he'd be hassling me - 'mum, when can we go in the pool?', because I wouldn't let them in until I was ready to watch them."

Mrs MacMillan said friends and family had been surprised by the cause of Jack's death and none had heard of shallow water hypoxia.

She said Jack had been involved in underwater breath-holding "contests" in the weeks before he died.

Mrs MacMillan was tending to Jack's sisters near the pool when she noticed he was motionless in the water.

"When I saw him he was at the bottom of the pool.

"At first I thought, is he just playing, mucking around? It was so quick.

"As much as you try and protect your child, that's how easily it can happen."

Royal Life Saving Society maintains a fact sheet on its website about hypoxic blackout and, in June, provided information to pool lifeguards through its Lifeguard Network Blog.

NSW branch operations manager Michael Ilinsky said targeted awareness campaigns were difficult because the extent of drowning deaths caused by shallow water blackout was unknown.

"Such deaths are normally recorded as fatal drowning and therefore it is not easily identified," he said.

"We become aware ... only when these contributing factors are recorded by observers leading up to the unconscious event.

"Because we don't know to what extent this is an issue we are unable to specifically target user groups or activity sectors.

Jack's family have set up a Facebook page called Shallow Water Blackout SWB to build support for their campaign.