Williamson teamed up with Brent Barclay and they amassed 25 points, two more than Canterbury among the seven teams competing in five races.
Williamson drove The Guardsman to win a race in the series and his brother, Matthew, won with The Smiler.
Dexter Dunn was the most successful driver in the series with 23 points.
Jaccka Justy gave Nathan his first win as a trainer.
He also trained Burano to win yesterday.
Williamson is in his second season as a trainer
The trotter Jaccka Justy was having his first start after qualifying on November 18.
Jaccka Justy is a 4yr-old by Continentalman from Janine Jaccka raced by Riversdale breeders Charlie and Ailsa Smaill. He is a brother of Jaccka Jack, winner of eight races for the Smaills from the stable of Murray Gray.
Burano, driven by Dexter Dunn, was having his second start for Williamson.
Burano won at Winton in April 2008 for Kevin Townley but did not race again until last month.
Vi Et Animo recorded his 14th win and eighth from 12 starts at Ascot Park when he overcame a 40m handicap yesterday.
The Canterbury team of John McDermott and Richard Sissons won a series for amateur drivers on three non-totalisator races.
Teams also competed from Canterbury, Waikato and Southland.
McDermott was the leading individual driver.
