Sonny Bill Williams is favoured to stay with rugby union and the All Blacks on a new flexible deal after the World Cup.

Williams, who is also developing a career as a professional boxer, is being chased by rugby union and rugby league clubs from around the world with rapidly-rising dollar offers.

However, his Australian-based manager Khoder Nasser was quoted in the Sunday Star-Times newspaper as saying the New Zealand Rugby Union (NZRU) was in the box seat to claim the prized signature when his current contract ended after this year's World Cup.

"Sonny will remain in New Zealand as long as the NZRU is fair and just," Nasser said.

"We've started negotiations and, if they are fair and just, they have nothing to fear."

The NZRU is expected to come up with an innovative offer, expanding on the groundbreaking four-year deal announced last week to retain fly-half Dan Carter, with a similar announcement expected this week to keep All Blacks captain Richie McCaw.

Carter's contract has a built-in sabbatical clause which will allow him to play in the lucrative European club scene, just as he did for French club Perpignan in 2008/09.

Any deal with Williams might have to include breaks to play rugby league, and he has already specified any deal he signs will have to contain the right to continue his boxing career.

Neil Sorensen, the NZRU's manager of professional rugby, would not discuss how negotiations with Williams were progressing.

"It's still too early in the process to be making a comment," he said.

Williams, a former NRL star before being lured away by wealthy French rugby union club Toulon, was drawn to New Zealand by a contract which allowed him to box as well as play rugby.

"We couldn't compete with Toulon. We were millions and millions (of dollars) apart and this was just one way of securing him," Sorensen said.

Williams has won his first three professional bouts, with a fourth scheduled for next month in a week when his Crusaders Super Rugby team have a bye.

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