The race was won by the experienced Canterbury swimmer Annabelle Carey (Jasi) in 1min 09.86sec.
Kenneally was second in a personal best time of 1min 11.65sec and Chloe Francis (North Shore) came third in 1min 12.44sec.
"The race was hard, but it felt good at the end," Kenneally told the Otago Daily Times from the QEII pool in Christchurch last night.
"It made all the hard training worthwhile."
Kenneally (17), a food science and physical education student at the University of Otago, gained valuable experience at the Youth Olympics in Sydney in January and put it to good use last night.
She was ranked fifth going into the championships but stepped up a gear to secure a podium spot.
Kenneally can expect another medal because her best event - the 200m breaststroke - is yet to be held in the championships.
She is ranked second.
It has been a good year for Kenneally, who won five medals - four gold and one silver - at the New Zealand age group championships in Wellington last month.
Kenneally is a member of the Otago Swimming Coaching Academy (Osca) coached by Gennadiy Labara. Matthew Glassford (Neptune), another member of the Osca squad, broke an Otago senior men's record when he finished fourth in the men's 100m breaststroke in 1min 05.10sec.
The race was won by Glen Snyders (North Shore) in 1min 00.57sec.
Glassford is ranked third in the 200m breaststroke with a time of 2min 20.80sec.
He won a gold medal in the event at the national age group championships.
Lil Clearwater (Neptune) broke an Otago girls-aged-15 record when she swam 32.13sec in the women's 50m backstroke heats.
Meanwhile, a rejuvenated Corney Swanepoel set the pool alight in a brilliant battle on the first night of finals in Christchurch last night, NZPA reports.
The 23-year-old from Swimming New Zealand's international training centre went under his own national record to beat off fellow Beijing Olympians Moss Burmester and Daniel Bell at the QEII Leisure Centre.
The meet doubles as the official trial for July's world championships in Rome, with swimmers having to meet the tough new Fina 900-point mark.
Swanepoel, Burmester and Bell all achieved the qualifying mark and set new personal bests, but with a limit of two swimmers per event, Bell missed out on qualifying for Rome.
Bell (18) later broke his own New Zealand record to retain his title in the 50m backstroke.
As it is a non-Olympic event he had to meet a 1000-point qualifying target and was just short.
But if he qualifies for the world championships later in the meet, he can swim the 50m backstroke as a second event.
Also through to the world championships is Beijing Olympic semifinalist Glenn Snyders, who broke his own national record in the heats yesterday morning.
He was marginally slower in the evening, winning in 1min 00.57sec but, notably, has produced the second and fifth fastest times in the world this year.
Swanepoel was thrilled with his win after a superbly timed performance, turning in 24sec and powering home in 51.61sec, with Burmester an encouraging 51.99sec in second and Bell third in 52.28sec.
"I had done absolutely no fly until five weeks ago. For the last five years I have been concentrating on the 100m butterfly race, so it can get a bit much after all that time," Swanepoel said.
In other finals, North Shore's Emily Thomas retained her 50m backstroke title in 29.10sec; Athens Olympian Annabelle Carey (Jasi) won the 100m breaststroke; US-based Kane Radford (Aquatix, Rotorua) took out the 800m freestyle; 16-year-old Ellen Quirke (Capital) claimed the 100m butterfly; while Taranaki's Charlotte Webby retained her 1500m freestyle title.
