The Australian is in his second season at Langtree Park, leading Saints to a top of the table finish, booking their spot at Old Trafford with victory over Catalan on Thursday night.
Saints Chairman Eamonn McManus stated: “Nathan received our blessing to apply for the Wests Tigers head coaching position and although not successful, the same family reasons for a return home still pertained. We therefore willingly agreed to grant him an early release regardless.
“Nathan has done a great job in rebuilding the St.Helens squad over the last two seasons and we are very strongly positioned indeed for the future.
“The Club’s focus is now entirely on winning the Grand Final at Old Trafford next week.
“We are confident and the team is looking forward to this game with absolute determination and focus. As it is Nathan’s last game in charge, there is even more to play for and the motivation levels are even higher.”
The former St George-Illawarra Dragons player and coach started his Super League coaching career in 2009 with Huddersfield.