If you are experiancing Sewer overflowing from your drains and you are not using any fasilities; it would be fairly likely the local Sewer main is blocked and the water from the main serwer system is discharging into your yard.  

Call Hunter Water on 1300 657 000 for a free service.

The overflow gully is designed to prevent sewer from backing up into the building every building should have at least one.
Re build broken boundary shaft

Re build broken boundary shaft