Kiwis coach Stephen Kearney says league officials and referees will have a tricky time successfully implementing the shoulder charge ban next year.

The Australian Rugby League Commission's decision to outlaw the practice has triggered outrage among some of the game's biggest stars, with Sonny Bill Williams - the most famous exponent of the shoulder charge - declaring on Twitter: "This is league, not tiddlywinks!!"

Medical experts, including Warriors doctor John Mayhew, backed the move to improve player safety, but several coaches were disappointed, including Wayne Bennett.

Kearney, a former tough-tackling back-rower who was no stranger to the judiciary during his playing days, said it remained unclear what did or did not constitute an illegal hit.

"I've seen the shoulder come into contact with the head in a normal tackle. Subsequently you get the same result as a shoulder charge, and they're going in to make a tackle," said Kearney from Brisbane where he is now an assistant coach with the Broncos.

"I guess my point is, are they going to outlaw that too?

"There are a number of tackles where the shoulders come in contact with the head, and you get someone being knocked unconscious. But was it a deliberate hit on the head? There is a lot of grey area."

The ARLC had reviewed a detailed report into shoulder charges and determined that the increased size of players was creating an "unacceptable injury risk".

The commission is hoping to extend the ban internationally, although New Zealand already has a domestic ban in place.

Shoulder charges are banned in rugby union.

Kearney said he had never discussed shoulder charges with his players while coaching, but it had often been used successfully as a defensive ploy.

"You look at every incident this year, and Greg Inglis had a couple that stood out, but again, the guy is six foot four," Kearney said.

"Someone has broken through the line, and he's going in there to try and stop the attack. I know if he was on my team and he'd done that, if he's trying to put his body on the line like that . . . I don't know."

The review found that shoulder charges made up 0.05 per cent of the 142,355 tackles made in 2012, with less than 4 per cent resulting in injury to the attacking player and less than 1 per cent to the defensive player.

"This is about reducing a potential risk of serious injury to our players," interim NRL chief executive Shane Mattiske said.

"The report shows that the shoulder charge is not a significant part of the game and its removal is not likely to impact on the way the game is played. With the increase in size and strength of the players, we believe this is the time to eliminate a potential risk."

- © Fairfax NZ News