"He waved his arms, struggling to stay above the water.
"Two of the boys in the group went to his assistance but he slipped from their grasp and disappeared below the water."
TJ was seen to "give up" and "start to panic", the coroner said.
His body was retrieved about 20 minutes later by a member of the public who dived in to find him.
The person described the water as "very murky with about half a metre visibility".
TJ's sister told police her brother was "happy" in the water, but not that competent, Coroner McElrea said.
Constable Robertson (eds: no first name known) told the inquest there was no "fooling about" before TJ got into difficulties.
There were 28 warning signs placed around the lake, including six signs erected six days before the drowning, that alerted the public that the lake was not patrolled and should be used at people's own risk.
Coroner McElrea noted none of the signs indicated how deep the water was.
"Unfortunately Tominiko (TJ) Palenato overestimated his capacity to swim the distance in question in deep water.
"In short, he did not have the swimming ability to achieve what he set out to do."
TJ may not have understood until too late that beyond the perimeter ledge the water would be deep and he would be out of his depth, he said.
He recommended the lake owner, Todd Property Group, and soon to be new owners Waimakariri District Council, review the water safety signs, including whether there should be extra signs to indicate the lake was deep and swimmers must swim within their ability.
Todd Property Group managing director Evan Davies said the company accepted the coroner's recommendations.
"The Waimakariri District Council is due to take ownership of Pegasus Lake early next year and Todd Property will provide any assistance the council might need to carry out the review and to implement any changes."
Waimakariki District Council was not immediately available for comment.
