I thought this was a bird (Martin) as it flew rapidly up to a small tree hollow then away, multiple times. So size is somewhat bigger than a Tree Martin.
I'm pretty sure not a megabat. I'm guessing a Gould’s Wattled Bat (Chalinolobus gouldii) because it was hawking over water, in full daylight and about the right size and colouring. Have put that in as an ID, but looking forward to an informed ID from someone knowledgeable.