A slow or blocked drain is a reason to ask a plumber, not a diagnosis of the whole sewer. Say which fixture is affected and whether the water drains slowly, stops, or comes back up.
What is useful
Note the room, whether other fixtures in the house are affected, and whether it followed heavy rain. Fats, oils and wipes are common contributors in a kitchen or bathroom line. A licensed plumber traces the blockage. You do not need to name the cause.
What to leave alone
Do not open an inspection point yourself. Sewer gas and contamination are the reason. If you have already poured a chemical cleaner, tell the plumber before they open the line. Those products can injure the next person and can damage fixtures.
If sewage is coming up inside the building, keep people away from the water and stop using the affected fixtures. That is urgent. It is still not a job for this website to dispatch. If there is immediate danger, call Triple Zero (000).
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