Dion Robbins died at his family's rented home in the Darwin suburb of Moulden in June 2025.
Despite CPR efforts by his father, neighbours, police and paramedics, Dion could not be saved.
He was an adventurous and active boy whose family would always feel the pain of losing him, his inquest was told on Wednesday.
Dion's father told police he had a headache on the day and fell asleep after lying down with Dion to watch a movie, counsel assisting Fiona Kepert said.
When he woke, Dion was gone.
The father found his son floating face down in the pool, and a tipped-over stool next to the latched pool gate.
Forensic examination later showed what appeared to be small footprints in the dust on the stool's seat.
Ms Kepert said evidence would show Dion was tall enough to reach the latch to open the gate if he was standing on the stool.
The inquest will probe pool fence safety after Ms Kepert said the NT needed a more robust compliance system in line with other jurisdictions.
"Hinges and latches wear out, especially in the Territory's climate, and they can fail without warning, she said.
Four months before Dion's death, an inspection of the pool fence at his family's rental home found rust damage, the gate was not self-closing and needed to be fixed by a contractor.
The Pool Fencing Unit inspected the fence 45 days after Dion's death and found multiple issues of non-compliance.
Since July 2002, 597 children aged four and under have drowned in Australia, just over half of them in swimming pools.
In the Northern Territory the proportion was even higher, with almost three-quarters of child drowning deaths in pools.
Investigations into child drownings have shown time and time again that failures in fencing and supervision led to tragic outcomes, Ms Kepert said.
When young children were in or near water, an adult needed to watch them at all times, as children usually drowned quickly and silently, she said.
"There is often no splashing and no cry for help. In most cases, the child has been unsupervised for as little as two to five minutes."
Chairs, stools, toys and anything else a child could climb should be kept well away from pool fences.
The coroner needed to consider whether the pool fence at Dion's home was functioning properly at the time of his death.
Whether or not the gate was working properly may not have been a cause of Dion's death, Ms Kepert said.
The coroner may form the view that the presence of the stool suggested Dion opened the gate himself, she said.
But Ms Kepert invited the coroner to address whether the NT's pool fence compliance certificate regime needed an overhaul, involving more regular checks on pool enclosures.
The inquest before Deputy Coroner Chrissy McConnel continues.
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