I'm assuming it depends on the situation, an attacking player diving for a try whilst in mid air compared to a player catching a ball from a kick who is in mid air.

In the first instance the defending player should not be penalised for trying to stop a try.

I heard that too. Sounded bizarre, as it doesn't seem right. I thought tackling a player in the air (ie with both his feet off the ground) whilst in the action of catching the ball, was a penalty. The action of tackling a player who already has possession and is diving to touch down is perfectly legitimate.
We'll have to wait for jonh or Grimmy to get the facts.

The NRL has has had this rule since at least last year

Aussie rule is that a player can be tackled in the air if he is from the side that kicked the ball.

It's an interesting one. personally I don't like the way that once a player has jumped into the air then by the letter of the law he can't be touched, leaving the defense pretty much helpless, other than to challenge for the ball themselves. It will make kicks into the corner a lot easier to defend if you can shove a player into touch before he's landed. Whether that's a good thing or not is up for debate Isuppose

i like the NRL version as it helps smaller wingers get a chance to stop a try