The answer is pretty simple I would have thought. Wait until they have been smoking for a while – then knock really loudly on the door. When they open it – red-eyed and slightly paranoid, calmly introduce yourself and let them know that recently the cops have been targeting dope smokers in the Mosman area – check the Mosman Daily crime round-up for details – they are always busting kids down at Clifton Gardens and being a good neighbour and all you just thought you would warn them. Then wish them all the best and excuse yourself. I doubt you will smell them after that. 