Exactly, but they have no easy way of direct metering domestic sewerage usage, so they do it indirectly via your household water usage (usually something like 7

Exactly, but they have no easy way of direct metering domestic sewerage usage, so they do it indirectly via your household water usage (usually something like 75% goes down the drains). There needs to be a second meter to "subtract" the irrigation usage from the household usage so you don't get billed for that (and if it's a separate reclaimed water supply, then there's no subtraction, it's just a separate line item). If you don't have the 2nd meter and your irrigation is running from your domestic water supply, then yes you're absolutely paying for sewerage you're not using.