Tomkins, 24, has been linked with a move Down Under for several months, and Wigan will receive a world record transfer fee for the England international, who was under contract at the DW Stadium until the end of 2016.
As part of the deal, Wigan will have first option to re-sign him should he ever decide to return to Super League.
He said: “This has been a very difficult decision to make but it is now perfect timing for me to make the switch to the NRL. It is something in my career I have always wanted to do and this agreement with the Warriors also allows me to return back to Wigan in the future.
“The Wigan club have been fantastic to me from day one and having progressed from a Community Coach to an Academy Player and then playing First Team was a real dream come true for me. I owe the Club a huge thank you for all they have done and I head to New Zealand with some great memories of Grand Final and Challenge Cup success and some very special times.
“Going to Auckland is a huge step for me and my Rugby League career but one I am really looking forward to. Before that though I want to end my time with further success with Wigan as we aim for Grand Final glory and then with England as we focus on winning the World Cup on home soil.”
Tomkins burst on to the scene in 2008, scoring five tries on his Wigan debut in a 106-8 thrashing of Whitehaven in the Challenge Cup.
He has featured in five successive Super League Dream Team’s and was the 2012 Man of Steel.
Warriors chairman Ian Lenagan said: “We are extremely disappointed to be losing a player of Sam’s talent and ability, but Sam’s determination that now is the time for him to be taking up the personal challenge of playing in the NRL, is the driver of this deal at this time.
“Sam is still only 24 years of age and is determined to return to Wigan in the future. If he does, he will do so an even better player than he already is and he will still be at the peak of his career.”
Tomkins has won two Challenge Cups, two League Leaders Shields and one Super League Grand Final during his time at the Warriors.