When they leak they can be a real pain, unless you’ve repaired a toilet before, then they can be a perfect way to blow a couple of hours on a Sunday morning.
If they leak into the bowl, in between flushes, then it’s probably the main washer/seal into the bowl from the cistern. I look like having to replace one of those in the near future, so I’ll leave that one for now. Many a time though, you will hear it fill, and fill and fill. If the water isnt going into the bowl, then it’s probably the inlet valve that allows the cistern to refill, and then turns off when it’s refilled to the correct level. The inlet valves are pretty easy to replace, so 2 x inlet valves have been replaced. The new ones (generic fit) are $19.95 from Bunnings and refill quickly, MUCH quieter and faster than the older ones (even if the old ones are quite new), so I didnt need much convincing to replace both of mine.
The little hoses that connect the water supply to the inlet valve (and cistern) were also replaced at the same time as the washers of those had gone also. Replacement hoses complete with washers are only $4.60ea, so for a total of around $25 per toilet, I can go out and know the cistern isn’t going to take an hour to refill or possibly overflow!
Back to the main washer between cistern and bowl – I’ve bought a couple of these over the years from Bunnings, and not one of them has sealed properly. If it “almost” seals (still a very slight leak into bowl), you can help it along by putting weights in the plastic central shaft. That helps put downward pressure on the seal and will often “fix” the leak. The better option is buying the replacement seal/washer from Reece Plumbing. It’s a different rubber compound and works better. Looks the same, feels the same, but the Reece ones work.