Rapana took to social media to thank Raiders coach Ricky Stuart, the fans and his ex-teammates after the Panasonic Wild Knights officially announced his signing on their website on Monday.

The 30-year-old said he felt like he had plenty of unfinished business in rugby union as he prepared to join the Japanese Top League team.

He's getting married in November and then heading to Ota, near Tokyo, after that.

The 30-year-old is returning to rugby union, having spent the past six seasons with the Raiders.

He played 113 NRL games with the Green Machine - on top of the five he played for the Gold Coast Titans in 2008 - and scored 69 tries while forming a feared right-edge combination with star centre Joey Leilua.

Rapana played 11 Tests for the New Zealand Kiwis following his debut against Australia in 2016.

Before that Rapana was on a development contract with the ACT Brumbies, but he never played a Super Rugby game.

Following the Raiders' grand final loss to the Sydney Roosters, Rapana floated the idea of trying to come back to Canberra on a short-term deal, but his Instagram post has put that to bed.

"Man oh man where to begin. Firstly I want to take this time to thank everyone at the Raiders and Ricky Stuart for the past six years," he posted.

"I am forever grateful for the gamble you took and the opportunity you gave me in what feels like forever ago in 2014.

"To the fans of the mighty Green Machine who have supported the club through the good times, but most importantly to fans who were there through the tough times.

"You guys made my time at Canberra so much more enjoyable and I will miss running out to that Viking clap every home game."

The Raiders have a ready made replacement for Rapana in Bailey Simonsson, who made his NRL debut this year after switching form the All Blacks sevens program.

Simonsson can play on either the right or left wing, as can Australia international Nick Cotric.

"It's has been an absolute dream come true and an absolute honour and privilege to put on that Kiwis jersey," Rapana said.

"I'm super excited for this next chapter in my life and I believe I still have a lot of unfinished business in the rugby union arena."