Williams recently left the Crusaders to join fellow Super 15 outfit the Chiefs on a one-year deal but that will be his last union contract.
The 26-year-old has reportedly agreed to join NRL side Sydney Roosters in 12 months time, five years after he swapped league for union in controversial circumstances.
Williams was widely condemned by the rugby league community for walking out on the Canterbury Bulldogs mid-contract to join Toulon in 2008.
After a slow start in union, he moved to the Crusaders and became a huge star back in his native New Zealand. He won his first All Black cap against England at Twickenham in November 2010 and was part of the World Cup-winning squad late last year.
Williams is banned from returning to any other rugby league team than the Bulldogs before 2013 but newspaper reports in Australia say he has all but finalised a five-year deal with the Roosters.
His agent insists no paperwork will be signed for a number of months but the Sydney-based Daily Telegraph reports that details of a recent meeting between Roosters chairman Nick Politis, Channel Nine TV bosss David Gyngell and Williams were leaked by Williams' representatives.
Williams is still attracting interest from big-spending French and Japanese union sides but any attempts to keep him in the sport beyond 2012 appear fruitless, especially as he has long been enquiring about buying property in the Sydney area.