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She was competing in the Wharenui swimming club's annual open meeting, one of the three top meetings organised by clubs in the South Island each year.
Williams (16), a pupil at St Hildas Collegiate School, swam a personal-best 1500m time of 17min 03.12sec, 18sec faster than her previous best time.
It was worth 818 Fina points and was her first 800-point long-course swim.
Williams broke Olympian Anna Wilson's 1993 Otago senior women's record by 0.83sec.
She also broke the New Zealand record for a 16-year-old, set by Commonwealth Games representative Rebecca Linton in 2002, by 5.05sec.
The performance pleased her coach, Andy Adair.
"It was an awesome effort because Phoebe is in full training," Adair told the Otago Daily Times from Christchurch.
"I'm really excited.
"It came completely out of the blue because Phoebe has been feeling flat since returning from the transtasman competition in Australia last month. I didn't expect it."
Williams swam 7950m at Moana Pool on Thursday morning in a training session that included fast sets of 8x400m.
She then drove to Christchurch.
Wilson was one of Duncan Laing's star pupils and competed at the 1996 Olympic Games in Atlanta.
"I have always said that when we started to erase the records of Anna Wilson and Danyon Loader from the books, we would be getting somewhere," Adair said.
Adair knew Williams was on target for a top swim when she passed through 800m in 9min 05sec, a time just 5sec slower than her best for the shorter distance.
Williams gained valuable experience and learned to adapt to all types of conditions during the transtasman series, in which she achieved six personal-best times from her seven swims and won a silver and two bronze medals.
Natasha Davidson (16), a pupil at Kavanagh College, was another member of the Waves squad to achieve a milestone in Christchurch when she qualified for the national age-group championships and the New Zealand spring championships for the first time.
Davidson lowered her best long-course time by 20sec when she was fourth in the 800m in 9min 52.01sec.
Georgia Raudon (12), another Waves swimmer, qualified for the national second division championships for the first time when she swam the 800m in 10min 52sec.
It was a personal-best by 1min.