To those complaining about Wayne Barnes, I would remind you of the penalty count: 12-6 to Argentina. Yes, the yellow card was undeserved, but so were the multiple scrum penalties against Australia when Herrera was boring in on James Slipper, so was the penalty for side entry against Kuridrani when the Argentine player dragged the ball back under himself. Apart from Barnes' whistle, the scoreline would have been much worse for Argentina.
What really sticks in the craw for us Wallabies fans is that Hooper was subsequently suspended for 1 match for a no arms clean out at the ruck, while Burgess got an off-field yellow for a similar level offence. To be honest, if Hooper and Burgess had both been yellow carded on the day and Farrell let off, we probably would all have been happy enough and none of this post match ruckus would have happened.
Both players deserved red cards here. Because Weeks was sent off alone, that meant the Rebels played the whole second half 15 vs 14. Giving Jannie a yellow card only ended up rewarding him and his team for starting a fight with an unprovoked hit to the back of the head. It arguably cost the Rebels the game.
No there was no clear obstruction and the defender was nowhere near the ball carrier. The Pek Cowan try was disallowed correctly because there was clear obstruction and the defender could have made a tackle.