I can only remember one occasion at Moorabbin where a student performed a circuit below 400 ft in normal operations. The tower was very quick to call and an incident report was lodged as a result.
Glide approaches, low level circuit practice etc. I've done a few of the latter at MB (500ft amsl), all with the O.K from the tower.
This used to be only available to aircraft arriving off the NDB (helicopters permitting) or have they changed to allowing low level repetetive circuits as with some other GAAPs.
As far as low flying aircraft happening on a regular enough basis to be a problem is absolute rubbish.
I'd be much more worried about the organised crime that occurs in the area, have seen some very interesting things from the circuit happen in the surrounding industrial estates. There's more likelyhood of a man with a gun visiting your backyard than an aircraft.