If the wall isn’t common property (which I find difficult to believe) then it is the other lot owner’s property and you should talk to them about fixing it.
I’m far from being an expert on Victorian strata law, but as a general principle, if a problem in property owned by another person or entity is causing damage in your property then they should fix it at their expense.
It’s not up to you to hire experts, apart maybe from an experienced strata lawyer.