BEFORE Sonny Bill Williams fled the Bulldogs to play rugby union for a French billionaire who has made a fortune from comics, a leading Sydney barrister prepared a legal strategy that could end in the worst nightmare for Australia's three big football codes.

If the NRL and the Bulldogs are successful in convincing a French court to order Williams back to play rugby league, he will challenge the NRL's salary cap, arguing it is an unreasonable restraint of trade.

Williams's lawyers will claim the Bulldogs can pay more than his existing $450,000-per-year contract but are prevented from doing so by NRL rules that create salary parity across its 16 clubs.

Should an Australian court find in favour of the New Zealand international, it would entitle the AFL Players' Association to challenge its salary cap, dismantling a club structure that has enjoyed competitive balance for 30 years and allowing billionaires such as Dick Pratt to plunder top talent.

The Australian Rugby Union also imposes "contracting protocols" that set maximums on what its four Super 14 clubs pay players, an arrangement that is even more vulnerable because it was reached without the input of its players' association.

Williams's legal team does not expect a French court, much less the French Rugby Federation or Toulon, the club that will pay him $3 million for two years, to recognise any claim by the Bulldogs. Nor do they expect the International Rugby Board, now effectively run by the French, to be sympathetic to a code that raided the once-amateur game for a century.

However, if Williams, who turns 23 on Sunday, is ordered back to the Bulldogs, his lawyers will make a counter-claim against the NRL, particularly its salary-cap rules on third-party agreements, arguing they have prevented him earning more money. While restraint of trade is not inherently unlawful in itself, it must be deemed no more than necessary to protect the legitimate interests of the game.

All NRL clubs are prevented from guaranteeing third-party deals, meaning a club cannot make up a shortfall if a contract collapses, as St George Illawarra found when an agent for the France-bound Mark Gasnier could not secure a deal with Channel Nine.

However, when energy firm Firepower failed to meet payments to Western Force players, such as Wallaby Matt Giteau, the ARU allowed the Perth-based club to secure replacement sponsors for the remaining years of their original contracts.

Furthermore, the ARU has an exceptional-circumstances clause that protects it against a Wallaby going to a rival code, allowing the NSWRU to top up Lote Tuqiri's contract when he was considering returning to rugby league.

Williams's legal team will argue the NRL's strict rules preventing the Bulldogs underwriting sponsorships at the time that his five-year contract was agreed prevented him earning more.

The Rugby League Professionals Association has entered into a collective bargaining agreement with the NRL embracing the $4m-a-year payment ceiling, making it more difficult for Williams to argue the salary cap is unreasonable. The NRL would argue the salary cap is justifiable because it guarantees employment to its 400 players and protects them from rogue billionaires buying the best talent.

But it is unlikely to come to this because the chances of gaining an injunction against Williams playing in France are remote.

Courts are reluctant to enforce contracts for personal services, meaning they don't like forcing people to do something they simply don't want to do. Damages is the court's preferred option.

Despite the NRL's sabre-rattling before the NSW Supreme Court seeking an order to freeze Williams's assets, it will be reluctant to join any action to settle, lest it be sending a signal to other would-be contract breakers that it can be bought off to preserve the sanctity of the salary cap.

More likely, it will be left to the Bulldogs to make a claim against Williams, or an action against his management and Toulon for inducing him to break a contract. This will take time. Sonny Bill could be back in New Zealand playing for the All Blacks by the time the court heard the case.

Rugby union's rules are solely designed to protect the code. Brad Thorn played for the All Blacks against the Wallabies on Saturday night, having previously played rugby league for Australia, winning an NRL premiership with the Broncos and a Super 14 championship with the Crusaders.

Sonny Bill seems similarly destined to have the best of both worlds.