The 31-year-old former international was released by Rovers on Saturday, just six months into a three-year contract, and is now thought to have signed a three-year deal worth around £1.6million.
Mason, who signed a two-year deal with Rovers with an additional 12-month option, made just six appearances in Super League before the club lost patience with the wayward star.
The club stopped paying his wages, citing a breach of contract after discovering the player had travelled to France to have talks with Toulon owner Mourad Boudjellal without their knowledge or permission.
The Auckland-born Mason, who won 24 caps with the Kangaroos, is following in the footsteps of another controversial former NRL star, Sonny Bill Williams, who walked out on Canterbury Bulldogs to sign for Toulon.
Although he has never played rugby union, Mason has revealed he is set to play for the Barbarians against England at Twickenham next Sunday and against Wales at the Millennium Stadium on Saturday, June 4.
"I'm just so excited to be playing rugby and I'm looking forward to the challenge," he told the Sydney Morning Herald.
"I've certainly jumped in the deep end in agreeing to play for the Barbarians without having any rugby experience.
"I've just got to learn how to play rugby in a hurry and how to speak French because I don't know a word yet and they say it's not that easy to pick up."
Hull KR announced the capture of Mason last September after fighting off competition from Huddersfield, claiming it to be the biggest signing in the club's history.
However, the arrival of one of the game's biggest names soon turned into a nightmare.
He was expected to arrive in England on the Tongan passport which would have made him a non-overseas player.
However, with continuing delays meaning he missed the start of the season, Rovers flew the former Australia and New South Wales star into England on his New Zealand passport.
It meant the club had to de-register Australian scrum half Michael Dobson, who was recovering from ankle surgery, in order for Mason to play as one of Rovers' five quota players.
He made an immediate impact with a man-of-the-match display on his debut in Rovers' 40-22 win over Crusaders but his form quickly deserted him and he was de-registered a month ago to facilitate the return of a fit-again Dobson.
Rovers have been investigating claims Mason may have even discussed a deal with Toulon after he had signed for them but before he arrived in the UK.
Mason, who rejected the chance to join Bradford on loan, reportedly met Boudjellal for a second time on Friday to complete the deal.
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