A surveillance camera then captured the boy falling into the pool at 6.32pm.
His father found him unresponsive seven minutes later and called emergency services.
The new information follows allegations Kiser was deceitful about the length of time Trigg was left unsupervised.
Both the 3-year-old and his newborn brother were being attended to by Kiser while their mother was out with friends.
In interviews with authorities, Kiser claimed he had only looked away from his older son for three to five minutes.
But the investigation found the father’s attention seemed to be “divided” and it was unlikely he was aware of Trigg’s whereabouts at the time of his death.
“The combination of these factors led to drowning, and a remedy to any of the contributing circumstances could have prevented the outcome.”
While the family owned Katchachild netting designed to child-proof the pool, it was not in use while Trigg was alone in the backyard.
The boy’s mother petitioned to censor the police report after recommendations were made for her husband’s arrest.
She sought removal of two pages, the details of which she said could be used to re-enact her son’s death.
Officials are yet to charge Kiser in relation to the drowning, noting a lack of evidence.