Thank you spades for going to the trouble of finding this excellent document. That is so kind of you!
It's clear this medal is intended to cover "all cases where the claimants have been actually under fire..." So it is possible, but very unlikely that he really fought and was wounded if he is not in this list. And there is no George Cannon there.
But...
There is a Charles Cannon. In the 3rd Waikato regiment, 1861-1866.
The thing is, I've recently discovered that George was born as Charles Cannon, and changed his name after a first spell in prison, prior to being transported for a second crime. My grandmother - who knew George's wife, her grandmother - told me his name was Charles. I thought she had made a mistake! (There is no birth record for a George Cannon in Great Berkhamstead, but there is a birth of Charles Cannon, correct birth year and same father's and exact siblings names as on George's convict records. Not 100% clear, but very convincing.)
Of course this Charles Cannon in the militia is not necessarily Charles/George Cannon. Why would he change his name back to Charles just for his military service? There is a military record for this Charles Cannon on ancestry.com but I'm not a member. Is there another way to access it?