Also, it’s likely such a major renovation (moving walls or creating new rooms) needs council approval.
General infomation from the HIA is here:
https://housinglocal.com.au/renovating/before-you-begin/building-permits.aspx
You generally do need council approval for “reconfiguration of internal space by moving or removing walls, new window and door openings”.
I would have thought too, considering that you most likely own the air space in your garage (and not the OC), your permission would have been required (as well as the OC’s) if more of your air space was going to be used by another lot.
There are things like implied easements which allow original pipes etc to protrude into the air space of other lots. But once in place, I don’t think these easements can be automatically changed or extended without the affected owner’s permission – especially is the change significantly affects the other lot’s property (air space).
It might be time to contact a strata lawyer.