Sam Whitelock has signed a long term contract with New Zealand Rugby after the rules were relaxed for overseason players. Photo: Muzi Ntombela/BackpagePix

WELLINGTON New Zealand Rugby signed forward Sam Whitelock to a long-term deal Wednesday, but only after the world champions relaxed their selection rules to let him remain an All Black while playing a season in Japan.

The All Blacks usually refuse to pick overseas-based players but such is Whitelock's importance to the team that the rule has been tweaked for the first time to keep the 108-Test lock in the fold.

"We have to be flexible when it comes to renegotiating with our leading players and that's what we have done with Sam," NZR chief executive Steve Tew said.

Whitelock, 30, is seen as a future captain, replacing Kieran Read, who is among an number of senior players leaving after this year's World Cup in Japan.

The two-time World Cup winner will be tasked with maintaining the All Blacks' standards after this year's tournament, much like Read did after the 2015 edition, when eight top players retired.

"His recommitment alongside a number of other key players gives us a strong foundation for the next four years," Tew said.

Whitelock will skip the 2020 Super Rugby season to play with Panasonic Wild Knights in Japan's Top League.

Such overseas sabbaticals have been granted before but only on the understanding that they will not be eligible for All Black selection.

However, Tew said Whitelock would still play with the team in 2020 "on the basis of his long-standing and outstanding service to New Zealand rugby and the fact he has signed a four-year deal".

Whitelock said he was looking forward to playing in Japan, where he will be reunited with his former Canterbury Crusaders coach Robbie Deans. 

"Ultimately, this decision allows me to take some time with my family to experience a new rugby environment in Japan, before coming back refreshed and ready for the remainder of 2020 and beyond," he said.