10 Dec, 12:23
Ireland's John Lacey will referee his first ever RBS 6 Nations match in February, while Alain Rolland is also included in the Elite Panel in what is his last season.
Williams demonstrated his explosive ball-carrying and offloading skills, and proved to be a massive influence at the back of Ulster's pack.
He quickly became a fans' favourite and his performances were also recognised by the media, as he earned seven man-of-the-match awards in his debut season with the province.
His contribution to the Ulster team was further acknowledged in May when he won two awards voted for by his peers - the RaboDirect PRO12 Players' Player of the Year and the Irish Rugby Union Players Association's Player of the Year.
Commenting on his decision to stay on with Ulster, Williams said: "It's always nice to stay where you're happy. My family and I have settled really well and it means I can go out and enjoy myself on the pitch. Everything has fallen into place and I wouldn't want to play anywhere else to be honest."
Williams believes there is more to come from the current squad and the New Zealander is determined to build on progress made last year.
"We're very excited for this season. The core of the group from last season is still here and after a few injuries last season, we've got boys coming back in who are now 100%.
"When you play the game you want to win silverware. We lost in the PRO12 final last year and it was very tough to take.
"Departing at the quarters in the Heineken Cup was gut wrenching as well, so I guess there's some unfinished business. It's a stronger squad now and hopefully we will have a better season."
Ulster's director of rugby David Humphreys was equally delighted with Williams' deal, stating: "Nick's level of performance throughout last season was instrumental in Ulster finishing top of the RaboDirect PRO12 and extending his contract for another two years is a huge boost to everyone associated with Ulster Rugby.
"He is a genuine entertainer and is someone who possesses the ability to inspire the players around him and get the crowd on their feet.
"Off the field, Nick's a tremendous personality to have around the dressing room and I'm delighted he's committed to Ulster until 2016."