The wall between your units is common property. That means not only that they have to fix it, but the lot owners have to allow them access to fix it.
If the lot owners refuse, the strata committee can take them to NCAT to get orders compelling them to allow access.
If the strata committee or OC refuse to go to NCAT, you can go to NCAT and compel them to take action.
End of the day, the wall needs to be fixed and the lot owner has to allow it.