Boomerang over Sydney. Video is authentic, but what is it?

The first 20 seconds of this video reveal a boomerang in the upper part of the frame. It’s light and hard to see, but very definitely there, and it is not a balloon. However, it appears to be flying rather than sliding in a more typical UFO pattern. Could it be a classified aircraft? Possible, but unlikely to appear over a large metro area. Graded A.