Hi, I’m a bit rusty on this, but water courses do come under environmental and planning legislation and I’m surprised your common property extends that far. Depending on the category of the water course (dependent on it’s size) there is usually a riparian protection zone, with width corresponding to category and size of the water course. And on council LEPs these are usually zoned environmental protection with restrictions on what can be done within the env protection zone. (Incidentally, early in the history of NSW governor King forbade clearing of riparian vegetation because ensuing damage and consequences of removing creek bank vegetation were already evident). I’d look on council LEP and check what it is mapped as, and what is allowed within that zone. Likely, it will need creek bank restoration works. If it’s just a drain, it shouldn’t be too involved, but if it’s an actual creek it might be part of an environmental corridor or a threatened ecological community (very likely in Fairfield) – even very degraded creeks fall under endangered community “Riverflat Forest” which could require native soils, native plants, geo matting, coir logs etc. You may need to hire a bush regeneration company to fix it for you. They are usually fairly cheap. If your building is less than 30 years old, conservation of this area was probably part of the original development consent and may be in the original plans.