Reef, most places don't have untreated sewerage going straight into the sea. the problem is when there has been heavy rainfall causing overflows or sometimes due to power or mechanical failure. Other things to watch out for is seepage due to septic tanks if you are in an area which still uses these (Huia, Cornwallis etc), or where Rural effluents can wash or seep into the sea from Dairy sheds or piggerys causing high BOD, faecal coliforms, ammonia and nutrients (N &P) (all things you don't want in your tank) Having said that, most people in Auckland use NSW in their reef tanks and have done so for a long time without seeing any ill effects.