There are several common causes of roof leaks:

Your Roof is Getting Old: Roofing Tiles and Colorbond Roof Sheets over time become damaged by water, dirt and UV Rays.
Poor Installation: Sometimes DIY or Handyman Repairs can cause your roof to leak.
Blocked Gutters: Leaves and dirt build up in your gutters and cause rainwater to overflow out of the gutters and will eventually rust them out.
Flashings: Leaks can be caused by faulty or damaged lead flashing or metal flashings.

Its best to call a licensed roofer to let you know whats wrong with your roof and the cost to make the repairs and fix your roof.