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 would appreciate an informed ID from someone knowledgeable.
Sminthopsis have conspicuous pale underside, white fur on top of rear feet, the underside of the hind feet is hairy at the heel and sides and the local species, S. murina has three granulated toe pads in a transverse row across the ball of the hind foot and no other pads.