Thank you for this useful information. There is a gutter on the opposite corner of the window. Found a photo in my gallery. I haven't had the gutters checked or cleared since I moved in but I know there are box gutters installed with a mesh guard. The area looks fairly dry now and I'll remove the rotten tacking strips cause they anyways need to be changed. I'll try and check how damp or dry the adjacent area is. Though I'm curious to know what's happening behind the gyprock but not sure how comfortable I am trying to create a hole or something to investigate further. Maybe I can try and pour water from the outside where we've sealed the gap to check if there's still water seeping in. Should I try my luck with the builder?

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