Dolphin achieved the sprint triple-double between 2005-2007 and Millar will now head away to the Australian national championships in Melbourne next weekend.
Louise Jones capped a memorable championships to complete the 200m-400m double with a decisive victory in the 200m from the defending champion Portia Bing.
Jones, 28, who is currently basking in the best form of her life, blitzed to a new personal best in yesterday's 400m and shaved 0.01 from her quickest ever 200m to stop the clock in 24.28s; 0.19 clear of Bing, her Auckland teammate.
Smit's double was also impressive given the strong calibre of the 1500m field with Commonwealth Games medallist Nikki Hamblin, Olympian Lucy van Dalen and the fast-improving Camille Buscomb in the mix.
But Smit finished in 4:17.95 as she pulled away from Hamblin during the final 100m, while Buscomb finished third.
"I'm really, really happy,'' Smit said. "I sat in for what was a slow first lap because of the wind before going to the front. I was hoping for the double but it wasn't easy because of the tough competition.''
Hamish Carson won the men's 1500m title after a dash of sage advice from his 91-year-old coach Arch Jelly, who told him to leave his kick till the final 100m because of the wind.
It paid off as Carson ran down Tasman's Julian Matthews in the home stretch as he clocked 3:46.23 to take a close victory.
"Arch is a very wise man. I knew it would come down to the last 100m. Julian has a very good kick. I'm really pleased I could win at my home track,'' Carson said.
Cameron French won the 400m hurdles in a personal best time of 50.16s ahead of Waikato-Bay of Plenty teammate Michael Cochrane (50.24s).
Wellington's Philip Jensen won his 20th national hammer throw title with a 61.85m effort as he finished ahead of the man he coaches, Ryan Tinkle, by 87cms.
In the field, Mariah Ririnui retained her senior women's long jump title - matching the exact same winning distance of 6.01m from 12 months ago.
