The crash happened in front of tens of thousands of spectators gathering for the Australia Day fireworks display; which fireworks are launched from barges on the Swan River.
Not sure if the weather had a hand in the aircraft performance here, it has been an absolutely stinker of a day, with Perth recording a maximum of 42°C at 14:09Hrs and the temperature still on 39° at 17:00Hrs, around the time of the crash.
News media is now reporting the pilot is deceased. Certainly didn't appear to be a survivable crash, and the aircraft has been totally destroyed. The Swan is quite shallow in the area, he would have impacted the river bed.