Scrum half Willi joined Gloucester ahead of the 2015/16 season from the Crusaders in Super Rugby.
Christchurch born, he racked up 50 caps for both Canterbury in the ITM Cup, and Crusaders in Super Rugby since making his debut in 2010.
The experienced scrum-half added plenty of quality to the position alongside Greig Laidlaw and Callum Braley in his first season at Gloucester and made 19 appearances, bettering that total in his second season when he also skippered the side frequently.
His form was reflected in a call up to England's training camp in May 2017 and Willi became club captain for 2017-18 making 20 appearances and scoring five tries.
Willi continued to skipper the side in 2018-19, making 17 appearances and leading the team to the end of season play-offs. His superb form was recognised as he was called up to train with England, made his international against Wales and earned selection to England's Rugby World Cup squad