National Rugby League chief executive David Gallop is not prepared to "open a Pandora's Box" and tinker with the $3.25 million salary cap in a last ditch bid to prevent star halfback Andrew Johns from switching codes.

NRL powerbrokers held an emergency meeting with Johns' manager John Fordham in a bold effort to retain the world's No.1 rugby league player.

Johns is believed to be tempted by an offer - reportedly worth more than $1 million over two years - to play five-eighth for Super 12 rugby team the NSW Waratahs and could make a decision as early as Thursday.

The NRL boss was hopeful Johns would stay with rugby league after the code's "great opportunities" were stressed to the classy halfback at the meeting also attended by John Hartigan - CEO of News Limited, a 50 per cent owner in the NRL - and a senior executive from Channel Nine, rights holders for the NRL.

Gallop hoped a media job package outlined would act as a "sweetener" for Johns.

But Gallop said the NRL would not set a precedent and stretch the salary cap to retain any player - including the in-demand Johns.

"We are not setting any precedents. That is one of the things that we are being very careful with, that we are not making any exceptions to our rules," he said.

"In the next few years you are going to see a lot of players off contract with their clubs, some of them will be players of Test status but others will be players just as critical to their club's fortunes.

"We are not in a position to open up a Pandora's Box in terms of the salary cap - it's too important."

Gallop did not believe it was hypocritical of the NRL to give the Knights permission to push forward with a third party agreement, involving a construction company, to top up Johns' offer.

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