A very common issue that is often due to things going down the toilet that shouldn’t. If the toilet is blocked by a hidden object you are best to call a plumber, the unit may need to be removed if there is not external access to the drain by form of an access cap on the outside of the property. Toilets may also be blocked due to an issue further down the line such a roots in the drain. This is often the case in older suburbs that have Earthenware drains, roots find their way in through the joins in the ceramic pipe and cause all sorts of issues. Drains can be unblocked and roots cut with the correct equiptment.