Sewage system odours inside can show a completely dry catch, fractured vent

http://photohistory.oregonstate.edu/works/half/the-importance-of-regular-plumbing-maintenance

Drain odours inside can show a completely dry trap, broken air vent, or partial blockage; Geelong properties with huge trees or older drain lines might need CCTV assessments to locate intrusions and avoid reoccuring smells.