The woman, who was competing in the mile swim on the final day of the Aotearoa New Zealand Ice Swimming Open Water Championships, was airlifted to Dunedin Hospital in a serious condition after she was seen struggling in the water by organisers.
Event officials noticed the woman’s stroke rate began to drop.
As they continued to watch, her stroke rate kept dropping.
The woman was pulled out of the water by lifeguards and paramedics in an inflatable rescue boat and assessed by doctors on shore.
A helicopter was called because the swimmer had taken on a "mouthful of water".
The swimmer was triaged at Dunedin Hospital and by mid-afternoon was messaging organisers and supporters, thanking them for their care.
She was released from hospital late Monday night and race organisers said she was "feeling well".
IISA chairman Roger Soulsby, himself a swimmer, said the incident was "a really fantastic case study of how things should be done".
From the medics, lifeguards and skippers on the water, through to on-shore communications, processes were followed and quickly acted upon, he said.
"The drama of having to bring in a helicopter is unsettling, so we’ve talked to all our swimmers.
"I think the main emphasis is we are fully aware of the sport that we are involved in and the risks that are involved."
Event director Susan Sherwen said competitors who watched the drama were "shaken but ... OK".
Ms Sherwen spoke to each swimmer individually and all were happy to carry on with the event.
