How Can Fraser Anning Be Expelled From The Senate?, page-204

"....However imo it's self defence, the bloke attacked him from behind, Anning's reaction was to defend himself by smacking him a couple of times....."

No doubt the kid should have been reported to the police for the egging.

But still the law makes a distinction between self defence and retaliation after the threat is over, Anning clearly was retaliating, not defending himself, which makes it assault. (The kids age could make Anning's position worse.)

As for the other guys, there is a thing called reasonable force, a good lawyer could easy argue the force was excessive in this particular case.
(I have had someone complain to the police about the force I used to apprehended them, the police looked him squarely in the eye and said, "He is allowed to use reasonable force.".)

A 17 year old would not face an adult court, so he is still a kid, regardless of his self awareness of his actions.

I'm guessing the kid would now get off with a warning because of his age and to avoid opening up a can of worms.