TJ and four young mates decided to swim from one of the 13ha lake's 11 beaches beside a waterfall to where others were playing at some wooden piles protruding from the water some 60m away.
About 40m from the beach, TJ, who lived with his family in the suburb of Linwood, got into difficulties.
In a statement of evidence to an inquest before Coroner Richard McElrea in Christchurch yesterday, Rangiora police Constable Paul Robertson described what witnesses saw.
"Tominiko called for help and flapped his arms, struggling to stay above the water," he said.
"One of the boys, Lvi Ahtong, went to assist Tominiko. However, Tominiko struggled and pulled the pair underwater.
"Lvi let go of Tominiko and another friend, Jordan Topai-Aveai, tried to hold Tominiko out of the water.
"By now, Tominiko was unresponsive and slipped out of Jordan's grasp and disappeared below the water."
Members of the public joined the search by free-diving to the sandy bottom of the lake.
About 20 minutes later, one of them, Paul Skerten, found TJ lying on the lake bed.
He was rushed to land by boat but could not be revived.
At the inquest, Paul Armstrong, development manager for Toddy Property Group, which acquired assets of Pegasus Town Ltd, including the lake, on December 3, 2012, gave evidence.
He expressed his condolences to TJ's friends and family for the "terrible tragedy".
The artificial lake, up to 6m deep in the middle, was developed as a public asset, with Waimakariri District Council due to take over ownership and management later this year.
Its former owners looked at a range of safety options when designing it, consultating the local council, police, and the Royal Life Saving Society.
Given its size, lifeguards were deemed impractical and buoyancy aids were discounted over maintenance issues.
Widespread signage was deemed the most practical and effective option.
There were 22 caution and no jumping signs when Todd Property Group took ownership. It then put up another 20.
The area was "very heavily signed" that day, Mr Armstrong said.
"Clearly the young man swam beyond his experience and underestimated what was involved," Coroner McElrea said.
He adjourned the hearing and will later give written findings.
"As I have indicated, the one area I may be particularly looking at is ... the importance of only experienced swimmers attempting what TJ did on that day."
The Pelenato family told the inquest that TJ went swimming with his sister every weekend and had taken formal lessons at school.
Asked by the coroner if, on December 15, 2012, TJ had simply wanted to be part of what other kids were doing, one family member replied: "I would say he saw the boys having fun on the other side and would've wanted to join in."
