I’m no expert but to do this yourself is asking for trouble down the line (unless you are familiar with the terminology). You need a belt an braces agreement that cannot be challenged by a subsequent owner when their roof collapses and asks you to pay half.
You need to give a strata expert your ideas and he then turns it into a bylaw that is enforceable subsequently. I’m all in favor of DIY generally but this is not just an a/c fitted to a wall, it’s the future maintenance of 2 million $ properties and a little spending now could save a fortune later IMHO.
Here is the NSW web site
https://www.fairtrading.nsw.gov.au/Tenants_and_home_owners/Strata_schemes.html