Williams (16), a pupil at Otago Girls High School, won a bronze medal in the 800m freestyle and was fourth in the 400m freestyle.
She swam 8min 45.70sec in the 800m - a personal best time by 1.58sec - to finish just 1.3sec behind winner Samantha Lucie-Smith (Wellington) and 0.54sec behind second-placed Jessica Ashwood (Australia).
She backed up by finishing fourth in the 400m in a personal best time of 1.71sec.
She swam her best 200m on the way through by 0.66sec.
Williams' 800m time was worth 818 Fina points and the 400m was worth 800 points.
Coach Andy Adair kept Williams in hard training to make sure she went into the series tired.
"It was an impressive start for Phoebe," Adair said.
"She will freshen up during the series."
The six-day series between New Zealand and teams from each Australian state is designed to put swimmers under as much pressure as possible to see how they cope.
"They sleep at a new hotel each night, have long bus travel and train and compete at unusual times," Adair said.
The team will next compete at Wodonga, on the border of Victoria and New South Wales, and Canberra.
