Do you want to be drinking pool water?
When topping up your pool you should never leave your hose submerged in the pool water as this is a cross-connection which may allow your pool water to be drawn back up the line. Always leave an air gap between the hose and the pool.
Where should the backwash water go?
Under no circumstance should backwash wastewater be directly discharged to the environment or to the stormwater system. The wastewater is extremely harmful to the environment and promotes weed growth in natural bushland areas. All backwash water should be directed to the sewerage system.