Drain odours inside can show a dry trap, fractured air vent

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

Sewage system odours inside can indicate a completely dry catch, broken vent, or partial blockage; Geelong residential properties with big trees or older sewage system lines might require CCTV inspections to locate breaches and prevent repeating scents.