Couple of things, YMBB is a Helicopter landing site YMMB is Melbourne, Moorabbin. I dont think you're wrong but if you're so sure and you havnt already, log the times that they do fly at these heights and use the webtrack software to prove it. If you witness the call sign and have evidence of an aircraft doing a circling approach well below MDA take it to casa.
It logs the aircraft types, there is a PA31 on its way in now. Why dont you compare his height with the recorded height and see how well your judgement is. Just for your own reference sake.