Dont relay too much on NCAT – they are sometimes clueless, or give out too general information. You do need to prove the basic facts. But the strata plan should show the wall having the window is common property, as is the window itself. The Owners Corporation has a strict duty to maintain, and repair, and replace its common property. You can represent yourself at NCAT, and if you can put together a good case, such as with photos etc, with copies of quotes for repair, NCAT should rule in your favour. The strata committee is not likely to contest this, as their case is very very weak.