After reading this question: Does a bathroom exhaust fan have to vent to the outside? I realised that the house I live in now, and everywhere else I have ever lived, everything just vents into the roof cavity, and the roof cavity has "whirly birds" on the roof to circulate fresh air through the roof cavity.
This includes two bathroom exhaust fans and the range hood in the kitchen.
I would say that the bathroom exhaust fans are original, or close to original, to the house, which is mid 1970's. This means that there's been 40 years of venting into the ceiling cavity. The rangehood is perhaps only 5-7 years old.
I'm no master builder, but I do not see any signs of mold or rotting in the ceiling at all, and when we purchased the property 3 years ago the building inspector explicitly stated that they found no moisture related damage in the roof space.
In the apartment that I used to own, we asked strata for permission to install an exhaust fan, and we were told we could do it on on the explicit rule that it only vented into the roof cavity, and did not exhaust outside (because that involved modifying common property). So until now, I never thought anything of it.
Is this something that I should spend money on changing?