New Zealand's longest lake swim was conquered by a marathon, almost day-long ordeal that induced hypothermia, even under layers of wetsuit, thermals and a kilogram of petroleum jelly.

Long-distance swimmer Ben Campbell-MacDonald yesterday said Saturday's 81-kilometre, 18-and-a-half-hour swim of Lake Wakatipu, from Kingston on the lake's southern point to Glenorchy on its northern tip, would not have been possible without the co-operation of the weather.

"I tried it three years ago but, because I get motion sickness when the waves get up in a lake, I couldn't complete it," he said.

"But on Saturday you could not have asked for better swimming conditions. The initial plan was to swim as far as Queenstown and see how it went, but with the weather we had, there was no real question of stopping halfway."

Because of the lake's notoriously frigid temperatures, executing the swim involved a 5mm wetsuit with thinned-down arms, neoprene boots and a 7mm hood, layers of thermals and a kilogram of petroleum jelly slathered on Campbell-MacDonald's body.

A "mothership" safety boat cruised ahead of an inflatable boat, which fed protein paste, hot potatoes, bananas and lollies to Campbell-MacDonald every half-hour.

"Yep, lots of lollies, Leppin and hot potatoes, which kept my core temperature up, and kept me going, so a huge thanks to all of my support crew for everything," Campbell-MacDonald said.

Despite such rigorous precautions against hypothermia, the condition set in near the end of the swim.

"Once you get to the top of the lake near Glenorchy you have the Dart and Rees River outflows coming directly into the lake, which drops the temperature by another few degrees. By that stage I was so close that I carried on, but I did get hypothermia, which we were prepared for."

Despite full preparation, a case of violent vomiting, coupled with hypothermia, meant Campbell-MacDonald was checked into Lakes District Hospital.

Yesterday afternoon he described himself as "recovering" at friend and "mothership" driver John Mahaffie's place.

"My arms are crazily sore – they're destroyed really, but I had a great night's sleep and feel a lot better than I did last night."

While he had a clear explanation of the execution of the swim, the motivation to attempt it was less clear, said Campbell-MacDonald, a 31-year-old National Health Committee policy analyst from Wellington.

"It's kind of difficult to justify really, but it's something I've wanted to do for a long time.

"In one sense it's a bit of a silly-bugger thing to do, but I think if you over-analyse the urge and motivation behind doing something like this, you'd end up second-guessing it.

"I can't really explain why I did it, but I'm really happy I did."