Williamson became the seventh player to score a double century for Northern and their total of 540 currently ranks as their fourth-highest in first-class cricket.
In Rangiora, Martin Guptill gave Black Caps' coach John Wright another reason to smile ahead of their impending tour to Australia as he smashed an unbeaten 121 for Auckland against Canterbury.
Earlier, the home side declared their first innings at 491-7 on the back of 171 from another Black Cap, Dean Brownlie, although the declaration was slightly bizarre as it left Andy Ellis stranded on 97 from only 70 balls and in sight of a century.
Brownlie's knock should ensure his name will be read out tomorrow (weds) when the team to tour Australia to play in a two-test series next month is announced.
It was the third first-class century of the Australian-born 27-year-old's short career and followed on from a solid debut in the recent one-off test match against Zimbabwe where he made 63 in the first innings.
Test stumper Reece Young also chipped in with a well-made 80 as he and Brownlie put on 171 for the fifth wicket.
Canterbury then had Auckland on the ropes at 72-5, before Guptill found support in the form of the big-hitting Colin de Grandhomme, who blasted a 95-ball century and finished the day on 117 not out.
With not a lot of batting left to come for Auckland, Canterbury will want to make inroads early on day three if they want to force an outright victory after their massive first-innings effort.
At the Bert Sutcliffe Oval at Lincoln, the clash between Central Districts and Otago advanced along at a steady pace as the southern side knocked Central over for 277 in reply to Otago's first innings mark of 207.
Wicketkeeper Kruger van Wyk top-scored for Central with a nippy 72, while Ben Smith made his fourth half-century in only his fifth first-class innings with a patient 52.
Former international all-rounder Ian Butler picked up four wickets with his right-armers for Otago.
Otago ended the day at 29-1 in their second innings, still 41 runs behind, with Craig Cumming and Aaron Redmond at the crease after Black Cap Brendon McCullum was caught behind for two from the bowling of evergreen seamer Michael Mason.