After waiting 3 weeks for a response from Telstra, I followed up today by calling their general switchboard on 1300 388 387 and after a few queries (and a short wait) I was put through to their Property Management Group. I turns out that their property is managed by Transfield, so I ended up talking to a Transfield operator. I got a reference number (41528299) which is the most important thing.
I didn't get a direct number for Transfield though - if I have to call them again, I'll go through the switchboard and ask for their direct line.
The graffiti is not offensive, so we'll see how long it takes them to clean it up. They've got my phone number, so I'm also interested to see if they call back when its cleaned up.