Winstone Wallboards Ltd. have a bulletin Guidelines for Repairing Gib Plasterboard Linings in Wind or Earthquake Damaged properties.
This is a 4 page bulletin which helps identify different types of damage and how the gib board should be repaired or if it should be replaced.
Make sure you are looking at their Technical Bulletin - Version 2, issued November 2010. It has been available at many hardware stores but was/is out of print. A copy can be downloaded from here.