In his findings released Thursday, coroner Peter Ryan said the boys had been playing in the water without any adult supervision, with "significant amount of surf and a strong undertow" in the water.
While beachgoers called emergency services, Gordon and Cowlrick swam beyond the breakers, but by the time they reached Marama he was not breathing.
The coroner said the officers' actions were "heroic", as they had no buoyancy or rescue equipment with them.
"Nor did they have the backup that surf lifesavers normally have when they attempt a rescue at a patrolled beach," Ryan said.
"They placed their own lives at risk in an effort to save another. The actions of these two police officers deserve the highest commendation."
Because of the roughness of the seas, the two police officers attempted to resuscitate Marama in the water until a rescue helicopter arrived half an hour later.
A paramedic assessed Marama and confirmed that he was dead.
Gordon described the swell as "about 10 foot in height and dumping with an offshore wind".
No autopsy was performed on Marama because his family objected and his death was recorded as "presumed drowning", based on the evidence.
Ryan concluded Marama died as a result of being swept out to sea in rough surf and being unable to swim.
According to the report, Marama, also known as Boombie, and Whetu would often go to friends' houses after school.
The twins' mother, Liana Te Pou, told police she thought her sons were at a friend's house on the day in question. The father told police the twins had not been at his house that afternoon.
A man who had been at the beach two days earlier told police he'd seen three young boys playing in the waves without an adult nearby.
Ryan said Marama's death was preventable and would likely not have occurred if there had been adult supervision.
"This is such an obvious and well-recognised factor in the circumstances of this death that making further comments or recommendations would serve no useful purpose," he said.
