I didn't see today's game, but Dan Lucas, the Graun's textist, reckoned that it wasn't a deliberate knock on, it shouldn't have been a penalty and it certainly shouldn't hve been a sin-binning. He also hoped that Joubert wouldn't be refereeing any more games. I did see Cuthbert's knock-on last week, and I would doubt that that was deliberate either.
Obviously, anyone attempting to intercept a pass is going to have to come from an on-side position, so if his judgment is slightly awry, the ball is bound to be knocked forward. Cuthbert's attempt last week looked to me like a guy who was genuinely trying to get the ball at full stretch, and he might have been successful if the Aussie guy hadn't tackled him when he was in the middle of the attempt.