Ms Fairbairn was accompanied from Eely Point in the water by her 21-year-old son, Finn, while her father, David Strang, was on hand at the finish line, the Otago Daily Times reports.
She began the swim near Makarora at 5.57am, battling her way into a southerly wind all morning.
As well as Finn, supporters Peter Jackson and Marjorie Cook took turns swimming alongside her.
The water temperature at the start of the day was 13degC, but by mid-afternoon, that had risen to 19degC and the air temperature to about 30degC.
It was a second crack at the swim for Ms Fairbairn.
The cold forced her to abandon her first attempt to swim the full length of Lake Wanaka, on Good Friday last year.
The temperature on that occasion did not rise beyond 15degC during her 12 and a-half hours in the water, and her support crew "pulled the pin" just 8km short of Roys Bay.
She recalled of that attempt she was not tired, but did feel as if she had forgotten how to swim.
Even though Ms Fairbairn, a professional swimming instructor, did not finish last year, she raised more than $6000 for the Darryl Fairbairn Memorial Fund, named for her late husband, which provides funding for youth counselling through Community Networks Wanaka.
This time, she had already raised $5605 for the cause before the swim was over.
