A small leak in the roof can lead to a large repair bill. As the rain water comes in it drops onto the insulation or straight onto the gyprock ceiling sheets. If it’s just onto the gyprock, it soaks through taking the dust with it staining the ceiling. If it is left long enough, the gyprock breaks down and eventually it needs replacing. If it is dripping onto the insulation, it can (depending on the type of insulation) cause the same problems, but with an added consequence, the water soaks into the insulation, increasing its weight many times over. When it gets heavy enough, down comes a section of ceiling.
Whenever you spot a ceiling stain appearing, get it fixed. It could be as simple as a dislodged tile or it could be the rafters have sagged over the years. What ever it is Fixit-Right Maintenance Solutions can either fix it or point you in the right direction.