I was at the airshow and witnessed the accident first hand - the point of impact was 400m from where I was sitting. This was the worst 5 seconds I've ever experienced, as I saw the turn tighten with what appeared to be the application of left rudder, then the airspeed dropped off and then over it went. I knew from the start of that turn, as soon as the wing went over there was nothing that could be done.
In response to some of the questions, as I was sitting at a different angle to the 3 videos doing the rounds I can say the caravan and the Mallard weren't close on the first touch and go - would have been 100-200m apart. The steep climbing turn from the caravan did get my attention though. The incident happened on the third attempted touch and go, and the previous two were both too high, so it did seem that the third attempt was done lower and slower, presumably to come out on final with less height and energy to perform the touch and go in the limited space between the landing area and the narrows bridge.
Also there were crowd lines on both sides of the shore as well as hundreds of boats to the east, restricting the space to make the turn to base and final.
The incident happened when the aircraft was flying East and was turning through north when it banked over, so the sun wouldn't have been an issue.
The other part I hadn't had any explanation of was the fact that both aircraft appeared to finish their sequence and left the display area via the standard departure over the narrows, then south towards Canning bridge. Then, I saw the mallard make a left turn, flying over South Perth and onto this turn where the incident occurred. Until the start of the turn, I was wondering why he had flown back.