To prevent that from happening, you can start with simple, cheap fixes that may help, or fast-forward to more costly but surer solutions.
Install a Tray. Change Habits. Check the Flapper. Insulate the Tank. Install a New Toilet. Install an Anti-Sweat Valve.

How do I stop condensation on my toilet cistern?

Warm up your toilet.

Insulating your toilet tank can prevent water from condensing on the surface. You can line the tank with an insulating kit, or you can cover the entire outside of the tank with a tank cover. If you can prevent the cooler tank from meeting up with the warmer air, condensation won’t occur.

Why has my toilet cistern get condensation?

Condensation is caused by cold water entering your toilet tank causing a drop in temperature on the outside surface of the tank. During humid weather, the tank temperature can be low enough to cause the moisture in the air to condense on the tank surface.