Glenn Snyders, chasing a third Olympic selection, led qualifiers in the 50m breaststroke in 27.29, only 0.2s outside his national record, with Porirua's Bronagh Ryan topping the women's heats in 32.47s.
The leading Paralympic stars, Sophie Pascoe, Mary Fisher, Nikita Howarth and Cameron Leslie, were also in action in the morning heats as they look to better the standard to be considered for selection for Rio in the evening finals.
There are 256 swimmers from 56 clubs competing at the Sir Owen G Glenn National Aquatic Centre at AUT Millennium, with heats from 10am and finals from 7pm through to Friday. Finals will be broadcast live on Sky Sport 3.