There is a difference between the structure of residential charges for water (service and usage charge) and wastewater (service charge only). There are two main reasons for this difference:
Firstly, the costs of providing wastewater services are predominantly fixed costs. That is, the cost of providing wastewater services remain mostly fixed regardless of how much wastewater is discharged into the network. A major reason for this is that, unlike drinking water networks, wastewater networks are typically built above the capacity required to deal with wastewater discharges alone. This is a design requirement to allow wastewater systems to minimise the frequency and scale of wastewater overflows during heavy rainfall.
Secondly, while there is a small portion of wastewater costs that are variable, wastewater metering costs are very high compared to water metering costs. Rolling out wastewater metering to facilitate usage charges would involve significant costs that would likely outweigh any benefits.