IMO, a very poor call.
A deliberate knock forward should be ruled on what the player actually does, not the situation or field position he is in.
There was no slap down; he put his hand in the way of the pass can came within an hair of it sticking. If he had done the exact same action while on attack, i.e. Australia running the ball out of defence 10m from their own goal-line, and with a Scotland defender outside him, would that still have been ruled a deliberate knock-on?
If Maitland managed to catch that, he is very quick... a length of field try was on the cards.