Tim Watkins from the local Sheriff's office said they believe the boy may have died from positional asphyxiation.
He said there was no trauma to the boy's body.
Locals were left reeling following the death.
One dad said: "It breaks my heart! Kids just playing and having fun."
"Breaks my heart, he probably had anxiety which made his breathing harder," another responded. "Poor family, this had to be devastating since he was only here visiting his grandparents."
The death comes just a year after a toddler died in Christchurch after being found unconscious inside a running washing machine.
Christchurch police said the pre-school aged child was found unresponsive in a front-loading washing machine, which had been turned on.
Police said the child's death was not being treated as suspicious.