Third in the final was Mackenzie Christie (Mount) ahead of Paul Moretti (Mimiwhangata) in fourth.
Going into the under-20 girls' division as heavy favourite, Williams lived up to expectations with a strong win in the final.
Second in the final was Bianca Sansom (Auck) ahead of Claudia Fraser (Piha) in third and first-time finalist Celeste Calway (Waihi Beach) in fourth.
The under-17 boys' division final was thrown into disarray in the first five minutes when Daniel Farr (Tara) was deemed to have interfered with his opponents and incurred an interference.
In what turned out to be the closest final of the day it was Farr that fought back to beat his opponents with a 9.42 point heat total, comprising 6.5 and 5.83 point rides.
Kettle finished second ahead of Manu Scott - Arrieta (Sandy Bay) in third and Harry Macintosh (Waiheke Island) in fourth.
Kea Smith (Mount) went one better than her result in the under-14 girls division with a win in the under-17 girls' final, scoring 10.37 out of a possible 20 points for victory.
Smith completed two clean backhand turns on a lefthander for a 6.83 - easily the best of the final.
Smith defeated Claudia Fraser (Piha) who placed second ahead of under-18 national champion Jasmine Smith (Gis) in third and Abby Falwasser - Logan (Gis) in fourth.
Mount's Kehu Butler continued his winning streak in the under-14 boys' division, making it two from two wins this year.
Butler easily won the final with an 11.77 point heat total.
Taranaki surfer Joel Clegg produced his best national result placing second in the final ahead of Shane Kraus (Piha) in third and Lee Ririnui (Mount) in fourth.
Mount's Elin Tawharu backed up her national championships win with another victory in the under-14 girls' division defeating Kea Smith (Mount), Tiana Breeze de Mooj (Mount) and Zhana Hutchieson (Waitara) in the final.