Timber Sagging is definitely not good news for any roof. In Perth, it’s usually caused by the age of YOUR timber supports, climate stress, leaky roofs, and/or a combination of warping and the effects of gravity over time affecting older homes. Poor quality timber is another usual culprit in modern homes.
Please be aware that any issues with sagging timber can do a lot of interior damage very suddenly, without warning. Any kind of roof sagging problem needs immediate attention. Failure of roof supports can be extremely dangerous to residents and cause catastrophic interior damage.