Can somebody please suggest a solution for fixing this issue....looks like design fault....house is 7 yrs old and hebel construction .
Welcome to the Bunnings Workshop community @Maddy7. It's wonderful to have you join us, and many thanks for your question about roof leaks.
You'll need to work out how the water is entering the roof space and wall. I'd suggest that unless you have had a huge rain event, either your gutters are blocked, or they or a flashing has rusted through. If it was a design fault, then every time it rains, the water would leak into the roof, and this damage would have occurred much sooner.
Roof leaks can be quite hard to diagnose unless there is an obvious hole, and sometimes it's best to call in a roofing drainage specialist to assess the situation. Considering the damage to your wall, which indicates that a substantial about of water has entered your roof and wall, I'd encourage you to enlist the services of a professional before the next rain event to resolve the issue.
If you do get up on the roof to take a look, I'd recommend having a read through this guide on How to use ladders safely and exercising extreme caution.
Please let me know if you have any questions.