The Rio Olympic aspiration for the country's leading marathon swimmers ramps up at the New Zealand Open Water Championships in Taupo this weekend.

Rotorua's Kane Radford is chasing his fifth straight title in the men's 10km New Zealand Open Water Championship on Saturday, which is also the qualifying event towards Rio.

The championships provide the lifeline for swimmers to progress to the final Olympic qualifying event in Portugal in June.

The New Zealand Championships form part of the successful annual Epic Swim which will attract about 650 swimmers including a significant number of internationals, competing in a range of open water swims from 100m for children to the 17.5km events.

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Swimmers who finish in the top five on Saturday and then in the top six at the upcoming Australian open water championships will be considered for the final selection race in Setubal where the first nine swimmers will be added to the Olympic field.

Radford's main competition is likely to come from Waikato's Troy Balvert, the experienced Philip Ryan (Waterhole) and Wellington's Liam Albery.

The women's 10km event will see a rematch between multi-national champion Charlotte Webby (Aquabladz, Taranaki) against defending champion Penny Hayes (St Peter's, Waikato). Hayes, who was a prominent triathlete, upset Webby to win the title in 2015 and has focused on marathon swimming for the past 12 months under coach Carl Gordon at St Peters Swimming.

Overall numbers continue to grow for the annual event, with the seventh year attracting strong international support from Australia, the United Kingdom, Canada and Tahiti.

With a wide range of distances, the Epic Swim caters for allcomers from beginners and younger swimmers in their first open water swim, and for those more experienced swimmers the longer distances are a continued challenge.

Racing begins 8am Saturday with the 1km Mid Epic, Epic 5km, children's events, with the 10km at 10am and 2.5km race at 1.30pm.

Sunday's racing is from 9am with the Masters 2.5km Classic and the national 5km championships.