Cara Baker has added the five-kilometre title to her 10-kilometre win on Saturday at the national Open Water Swim Championships on Lake Taupo.
The 20-year-old, formerly of Manawatu but now based on the Gold Coast, out-kicked a group of four swimmers in a strong performance to claim the title.
The men's race produced one of the closest finishes in many years with just 4/10ths of a second separating the first four swimmers in a superb race.
Dunedin's Bryn Murphy narrowly pipped Rotorua's Kane Radford with Saturday's 10-kilometre champion Philip Ryan of Waterhole third.
New Zealand swimmers must finish in the top four at the Australian Championships at Penrith in March, or within 20 seconds of the winner, to qualify for the world championships in Shanghai - where the top 10 will earn automatic selection for the 2012 Olympics.