Dolphin said he was impressed with Millar's progress, particularly in the 100m.
"He is just a young bloke and is certainly running very well," Dolphin said. "He just needs to get some more training under his belt. In the 100 you are running against some big, strapping lads, which typically New Zealand guys aren't.
"If he strikes that balance between getting bigger and stronger, and maintaining that speed, then he will be on track to go places."
Millar and Harman will contest the Australian championships this weekend in a final attempt to run a Commonwealth Games qualifying time in the 100m and 200m.
Other Tauranga athletes performed outstandingly well at the nationals.
Mariah Ririnui was the only woman to jump over six metres in tough cross-winds, as she retained her senior women's long jump title. She also won gold as part of the Waikato/Bay of Plenty 4 x 400m relay team but was unable to defend her 100m title, finishing third.
Michael Cochrane won the 110m hurdles title and was just .08 seconds from winning gold in the 400m hurdles. Teenager Mike Goldie, in his first year in the senior ranks, was fractions of a second from winning a bronze medal in the 200m final won by Millar.
Tauranga coach Todd Blythe had a trifecta of success in the junior girls' sprints. Abby Goldie won gold in the 100m, 200m and 4 x 100m relay finals, with Amy Robinson winning golds in the 400m hurdles, long jump, 100m and 200m relays, as well as silver in the 200m.
Mollie Florence finished second behind Goldie in the 100m.