Williams, starting the match at centre, was not the only player to shine -- Colin Slade had an immaculate game at first five, running his backline line well besides kicking two penalties and three conversions while fullback Sean Maitland was industrious throughout and justly rewarded with a try.
Otago led 6-0 after 13 minutes from two Glenn Dickson penalties.
Canterbury hit back from a turnover. Williams and Slade combined to break the Otago defence on the right and when the ball was moved wide, hooker Steve Fualau powered over on the left flank for a converted try.
A cheeky dropped goal by halfback Andy Ellis stretched Canterbury's lead out to 10-6 and then second five Ryan Crotty dotted down after a beautifully worked move in which Williams was prominent.
Slade converted for Canterbury to take a halftime lead of 17-6 into the dressing rooms.
Otago's dressing room talk would have centred on how to stop Williams who relished the extra room and time the centre role afforded him to showcase his delicately executed off-loads which put teammates in gaps.
Canterbury were held scoreless for 18 minutes from the restart as little errors and stout defending combined to deny them.
Slade broke the drought with a penalty but as they have often this season, Canterbury faded out of the game and Otago struck twice in quick succession with two converted tries and tie the scores at 20-20. Slade was caught five metres out trying to run the ball out of defence as Otago made a rare foray upfield and scored a from the turnover through Joe Hill in the 62nd minute.
Two minutes later Dickson delivers a short pass to prop Halani Aulika who crashes through from five metres.
Williams then showed all his power and pace in a swerving 60m run which turned the Otago defence inside out before he went over for Slade to convert.
Slade kicked a penalty in the 78th minute and Maitland finished off a brilliant solo effort in the last play after the hooter went to give Canterbury an invaluable bonus-point for their fourth try of the game.
Result:
Canterbury 35 (Steve Fualau, Ryan Crotty, Sonny Bill Williams, Sean Maitland tries; Colin Slade 2 pen, 3 con, Andy Ellis dropped goal) Otago 20 (Joe Hill, Halani Aulika tries; Glenn Dickson 2 pen, 2 con). Halftime: 17-6.
