Nina Eades, 14, raced with poise making three backstroke finals and personal bests.
She placed sixth in the 50m with 32.13, eighth in the 100m with a 1:09.10 and ninth in the 200m, at 2:30.64.
Couper and O'Keeffe were ninth in their 50m backstroke age-group final as Couper got a 32.86 while O'Keeffe went 32.97.
"O'Keeffe also set a new 50m freestyle time of 28.97, narrowly missing the closely fought top 10," said Lowe.
In breaststroke events, 15-year-old Jacob Smith showed his potential in the 200m, breaking Aaron Bunker's Wanganui record by three seconds in the heats and trimming it again with a 2:36.35 for sixth in the final.
Smith also recorded a personal best in the 50m of 34:57.
Fourteen-year-old Fergus Smith had a busy meet, his best being in the 50m butterfly when he placed eighth from a sharp 28.77, following up with a 1:04.32 and ninth in the 100m butterfly final.
Thomas Ireton, 17, was a finalist in the three backstroke events, setting a new Wanganui record of 29.08 in the 50m, to place seventh.
Ireton was also sixth in the 100m at 1:02.07 and eighth in the 200m with a 2:16.29.