Why do my drains smell worse in summer?

If the smell is new and seasonal, check the traps in any little-used bathroom, utility or outside tap gully first and run water into them.

In hot weather, a partially blocked run holding standing waste will smell even when it is still draining. Jetting the run out removes the source.

A smell that persists after both those steps usually indicates a defect releasing gas outside the pipe, which needs a survey.