Pool & Spa plumbing sizes and names can be confusing, as plumbing sizes are referred to as "Nominal" sizes. "Nominal" means "by name only" and does not reflect the actual dimensions of the pipe. This is because pipes were originally measured by the internal diameter, where they slipped over a fitting. With time, the wall thickness became more important, and the name stuck or evolved.
Below are some charts with names over the actual dimensions. In Pool & Spa water plumbing, we only use "Pressure Pipe," not DWV or other plumbing sizes.
PVC Pressure Pipe ASNZ 1477 Dimensions - note that not all pool or spa fittings are exactly these measurements, as they do not necessarily adhere to Australian Standards.
Hose & Tube Dimensions