Code-swapping star Sonny Bill Williams and Wallaby Quade Cooper will headline a 'fight night' in Australia next month in a line-up that also includes rugby league player Paul Gallen.

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Williams, who returned to the All Blacks this year after a two-year stint with National Rugby League's Sydney Roosters, will take on American Chauncy Welliver in the January 31 event in Sydney.

In his last fight in February, Williams won a 10-round heavyweight bout against veteran South African Frans Botha.

The former World Cup-winning All Black star, in only his sixth professional fight, won the points on all three of the ringside judges' cards against the vastly experienced but ageing South African.

Cooper's opponent is as yet unnamed, but the fly-half had an impressive fourth round knockout win in his second professional fight in January. The 26-year-old floored former cage fighter Warren "The Fortress" Tresidder, 40, to keep his win record intact.

Cooper welcomed the Australian Rugby Union's approval for the off-season fight, saying he believed boxing kept him fit and as competitive as possible.

"Every player talks about ways of getting extra percentages — whether it be half a percent, one percent or two percent, and I think boxing is my edge," he said.

Williams said he had also sought permission from All Blacks coach Steve Hansen. "The major concern for myself was getting his approval," Williams said. "He said that as long as I don't get knocked out, he's happy with it. So hopefully that's the plan." Cronulla Sharks player Gallen's opponent has not yet been named.