Sonny Bill Williams in beast mode at the gym

SBW's idea of fun at the gym involves insane core reps. Photo / Instagram

Rugby superstar Sonny Bill Williams has posted footage of an impressive workout on Instagram.

Williams was recorded smashing out reps of front lever bar core exercises at Les Mills in Auckland.

He posted the video with the caption: "A little bit of fun today #gettingthereslowly"

In his first appearance as an All Black since the 2015 Rugby World Cup final win, Williams will start in tomorrow's test against Manu Samoa in Auckland.