I live under the western circuit at YMMB, and frequently observe the traffic inbound on the 070 radial. These guys pass almost overhead my home usually at 600-700 feet. Nothing unusual about that.
Apart from that, I have only rarely seen any fixed-wing below circuit height, and it was always due to low cloud base.
So I find it amazing that PaulDamian has seen someone at 250 ft on downwind !! More likely they had engine problem and were on final !!
And I can usually read the rego under the wing from 1000ft below, so to use reading the reg as evidence of low flying is utter crap.