The NRL has defended its decision to afford Cameron Smith and Johnathan Thurston a testimonial game in next year's pre-season, releasing a 10-point criteria including off-field and integrity considerations that indicates only Billy Slater would also be eligible among current players.

There is discontent among some rival clubs about the Smith and Thurston exhibition being a way of paying them outside the salary cap and setting a precedent for others to follow suit.

Cronulla coach Shane Flanagan on Friday suggested the trial between Melbourne and North Queensland at Suncorp Stadium would "open a can of worms" as the future Immortals would receive proceeds from gate takings and other revenue associated with the game that would not count towards either side's player payments under the cap.

Flanagan suggested the Sharks' veteran captain Paul Gallen would also be a fitting recipient of a testimonial after 17 seasons in first grade at Cronulla.