Event director Scott Rice has seen a lot of McEwan's performances as a commentator at national swimming events and said she has huge potential.
"I've watched her grow and develop into a very good swimmer who handles the pressure extremely well and who simply loves to swim, and to win."
McEwan's success on Saturday came in the absence of unbeaten Nelson teen Abbey Smale, who leads the series.
Smale will race in the final event, the Harcourts Cooper & Co Swim the Bridge in two weeks.
That 2km double crossing of Auckland harbour will decide the men's and women's series.
Alex Dunkley won the Mount Swim, finishing almost two minutes ahead of evergreen former Commonwealth Games swimmer Brent Foster.
Rice said Dunkley is another to keep an eye on.
"He's really impressed me and he will only get better as he gets stronger and more experienced."