What are the most common reasons a private drainage system fails to comply?

In our experience, the five most common compliance failures are:

A septic tank discharging directly into a ditch, watercourse or highway drain. Septic tanks must discharge to the ground via a drainage field.

A system that has failed due to a lack of maintenance – for example, a blocked or collapsed drainage field, damaged tankage, or ingress of rainwater into the system.

A tank that is significantly undersized for the current property, typically where bedrooms have been added without the foul drainage system being upgraded accordingly.

A system that requires a permit – for example, where discharge exceeds 2m³ per day to the ground, or the drainage field is within 50 metres of a private drinking water supply.

A system where the old septic tank has been replaced by a package sewage treatment plant, but the effluent was directed into plastic rainwater crates rather than a properly designed drainage field or watercourse – which is not a legal discharge point.