We treated mould and damp behind the wallpaper of an older house in Gladesville, after water from a failing box gutter in the roof got into the wall.
Nobody noticed anything until the mould showed in the top corner, which is how this kind of leak usually gives itself away: the water works through behind the wallpaper long before anything shows on the surface. The area was prepped and treated with Dulux mould stain blocker, which seals in the staining and stops the mould coming back through whatever goes over it. The leak itself came from the box gutter, so the roof repair is what decides whether the wall stays dry.
The wall hasn't been finished yet. It's being left as it is to see whether the roof repairs have worked 100%, because a wall closed up too early only hides a leak that's still active.

