The Highlanders loose forward will join Top 14 club Castres for the upcoming season on a two-year deal.
The 25-year-old made 10 appearances, mostly off the bench, for the Highlanders last year.
However, he was used more sparingly this season among a strong loose forward contingent.
Ben-Nicholas was one of four players the Highlanders confirmed yesterday would be heading overseas.
Japanese loose forward Kazuki Himeno was included in the social media post, an indication the star import may not be back next year.
He has linked with the Japan team to face the British and Irish Lions.
Otago utility back Michael Collins and Southland prop Siate Tokolahi were the other two, although both had been reported earlier in the year.
Collins is heading to Wales to join Ospreys on a two-year deal, where he qualifies as a local through his grandfather.
It will be the 28-year-old’s second stint in Wales, having also had a year with the Llanelli Scarlets at the end of 2015.
Tokolahi is also heading to France where he will join Top 14 side Pau.
