In the ACT at least, the address for correspondence of an owners corporation is a matter of public record at the Land Titles Office. I suspect the manager is being lazy.
I would either 1) jump up and down and insist the manager acts on the direction of the EC to discover the correct address (There might be a small charge for a title search) and write the letter, or 2) do it myself because it is less effort than (1), or (3) going next door and asking to chat to someone on their EC may well be the easiest.