The international, NRL & SL ruling is:
Even if a defender has dropped off the tackle then it is still deemed to be a two man tackle if a second defender gets involved with the ball carrier.
In this case though the kiwis had two defenders in contact with the ball carrier.
A penalty try is awarded if, in the view of the officials, the ball carrier would have definitely scored had it not been for the illegal play of a defender.
McGuire is about 18inches above the in goal area and travelling forward with the ball in both hands.
Now unless there is some suspension of the laws of physics in NZ there is no way that McGuire would not have scored that try without Webbs hand hitting the ball.
Penalty try!
As stated earlier of you deem that webb has accidently made contact with the ball then you can't rule knock on, so play continues and McGuire grounds the ball.
Try!
How the hell did the Video ref get scrum down Head and feed to NZ?
Simpkins positioning is superb and he should not have needed to refer it to the VR