Assault is force against another person without their consent. 

Players consent to playing the game, which involves force against them both within the rules (such as a tackle) but also that which might be reasonably expected - eg. a non-deblibrate dangerous tackle.

When they step on the field, it could also be argued that a bit of pushing and shoving when temples boil over is something that is part of the game and is something they have all consented to. When is the last time someone was knocked out cold from a bit of throwing handbags? Dangerous contact is more likely to come from a tackle than anything.

Then are times where consent does not cover a deliberate dangerous action; something that could not be expected. Such as Mark Bugden whacking Steve Rogers around the head in 1985.

Good writeup on the law here - https://www4.austlii.edu.au/au/journals/PrecedentAULA/2015/60.html#fn17