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About 430 swimmers are expected to take part in six different events, ranging from a 10km marathon swim to a 200m children's dash.
That number will eclipse the record-breaking 406 who competed in last year's event.
Races begin at 7am and finish about 11am, and organisers Eddie Spearing and Jamie Norman are hoping they will be able to beat the forecast rain and gusty northwesterlies.
"I reckon we're going to have a window of opportunity in the early morning. The wind is supposed to ramp up in the early afternoon, but we're wrapped up by 11am," Mr Spearing said.
"Two years ago there was a similar forecast and we managed to get the event through nicely before the wind came up. We're hoping for the same result this time," Mr Norman said.
A group of children with learning disabilities had also been given the chance to take part.
The group had been regularly swimming in the Wanaka Pool and Mr Spearing said their inclusion was "a no-brainer".
"It's good for them, they get so much out of it, and it's no difficulty for us."
The group will swim half of the Little Gems course, around 100m in length in shallow water.
Despite cooler temperatures all summer, Mr Spearing and Mr Norman said the water was only marginally colder than usual, about 17degC, compared with 19degC this time last year, and would make no difference to swimmers.