I have water getting into my Granny flat walls and its because the garden outside is too high and hence is spilling into the wall and water logged the internal gyprock. I have stripped back the skirting to find the gyprock is rotter at the base. I thought to fix it and get out of cheap before tennanting is to use MDF wall panels that go half way up the walls and cut out rotten gyprock and recover with boards, repaint and Bob's your Uncle and Nelly's your Aunt.

Providing I get rid of the garden along outside of wall and lower ground level the water shouldn't get in.

Any thoughts and ideas would be appreciated

I think everyone's having a shudder about this problem. You might have a never ending story with this job. Get rid of the garden, then probably the external sheeting, then possibly some of the frame work and re-fasten it down. By the time you do that you could well have to re-sheet inside from top to bottom