They clocked 49.07s to win by a comfortable margin from a Regional Australia quartet timed at 51.58s.
Small was also in the winning team in the 800m open mixed medley relay.
Again she was the lead-off runner, followed by Austen Heuvel, Louise Jones and Bailey Stewart.
They finished in 1min 36.31s and in second place was Fiji A in 1.38.95.
Chapman and Plimmer joined Christine Ashton and Mikaela Jefferson in taking the gold medal in their 4x100m relay in 50.73s with Chapman running the second leg and Plimmer the fourth.
They were well clear of French Polynesia who clocked 52.69s.
It was the second gold medal at the championships for Plimmer who had won the women's under-18 long jump the previous day.
Kruszona had a best throw of 48.32m to place third in the hammer throw behind fellow Kiwi Lauren Bruce (55.10m) and Australian Taylor Jade Minslow (49.01m). Katie Smith, also from Wairarapa, had been the bronze medallist in the women's under-18 shot put on the opening day.
Another of Wairarapa's burgeoning athletic talent, Alex Howden, was a second recipient of a Wairarapa Sports Education Trust grant and also benefiting was outstanding swimmer Joe Hammond.