Every apartment block in NSW is legally obliged to have a letterbox for the Owners Corproation. Send them a letter and ask them who you need to contact to deal with some (possibly imagined) issues like storm water drains or a planning application for a motorbike repair garage or packs of wild dogs roaming the area.
Alternatively, wait till an apartment comes up for sale and ask the real estate agent who the strata manager is so your solicitor can do a search.