Main has closed to within 0.44sec of the national record set by Wellingtonian Gareth Kean five years ago.
New Zealand's four other swimmers in action on Monday failed to advance past the heats.
The best of them was Emma Robinson, 11th quickest in the 1500m freestyle in 16min 25.78sec.
She was three places and 5sec outside a berth in the final.
Double Olympian Matt Stanley was a disappointing 29th fastest in the men's 200m freestyle, in a field of 73, while Natasha Lloyd had the same outcome in the women's 100m breaststroke.
Freestyle specialist Gabrielle Fa'aumausili was ranked 26th among 59 women in the 100m backstroke.