Stormwater should only contain clean rain water and not pollutants, such as rubbish, waste water, heavy metals, oils and sediment. 

Anything carried with the stormwater such as litter, soil, garden waste, car washing chemicals, car oil and grease from roadways, duck droppings and fertiliser also ends up in the creek, river or sea. This results in pollution of these waterways that can kill fish and other aquatic life, create unsafe swimming conditions and make the waterways in our parks, recreation areas and beaches very unatractive.

The sewerage system disposes of toilet and grey water (laundry, bathroom & kitchen waste) as well as wastewater that has been authorised by the South Australian Water Corporation (SA Water) to be discharged into the sewerage system. This water is treated at Waste Water Treatment Plants before being pumped to the sea. Some wastes, such as oils and solvents, cannot be treated by the sewerage system and must be disposed of by a licensed waste contractor. 

some sources of  pollutants are things like... Washing your car on the streets using soap that is not biodegradable. The soap can wash down the drain. leaving dog droppings on the side walk. On a rainy day, the droppings can go down the drain.