Having already reached the world championship qualifying mark in the 50m butterfly and 200m individual medley, the Coast club swimmer found the effects of a big campaign a step too far in chasing a third qualifying swim.
The 22-year-old streeted the field to win in 4m 46.66s, which was just over 3s outside the world championship mark. Gasson was half a length ahead of teenagers Ellie Eastwood (United) and Ruby Matthews (Otumoetai).
Ashby, who broke his national record in meeting the qualifying mark in the 200m individual medley, produced a brave effort to win the 200m butterfly.
The Fairfield club swimmer came from behind to take charge in the back half of the race, winning in 2m 00.19s ahead of top qualifier Shaun Burnett in 2m 00.98s and his North Shore clubmate Wilrich Coetzee third in 2m 01.52s.
Fa'amausili (United club), who qualified in the 50m freestyle, took out the 50m backstroke title, the event in which she won the junior world title. The 17-year-old won in 28.60s, 0.08s off qualifying for a second world championship event.
The champion over 100m and 200m backstroke, Bobbi Gichard (Howick Pakuranga) was second in 28.89s, with Wellington's Ella Cossill (Capital) third in 29.55s.
American-based Cantabrian Natasha Lloyd, who won the 200m breaststroke title, added the 100m breaststroke crown last night. She won in 1m 10.05s to hold off Ciara Smith (Northwave, Whangarei) and Bronagh Ryan (Porirua). She will chase the clean sweep of national breaststroke titles with the 50m today.
Hokitika's George Schroder had to fight off a stern test to claim the men's 100m breaststroke, winning in 1m 02.31s.
He was pipped in the 200m breaststroke final on the second day by Hawke's Bay swimmer Julian Layton but last night held on to win by 0.3s from Syrian visitor Azad Al-Barazi, with Layton a further 0.1s behind in third.
Howick Pakuranga's Bayley Main, who is on a scholarship at the University of Florida, won his first open title in claiming the men's 50m backstroke in 25.94s ahead of clubmate and 50m freestyle winner Daniel Hunter in 26.07s, with talented North Shore teenager Finn Kennard-Campbell third in 26.14s.