I've used a few around the city for private sessions. Yeah, all of them require a proof of ID to get the room keys. You usually get 2 keys, so you don't really need to show the girl's ID if that is difficult to do.
You will, however, have to meet her outside the room to give her the key if you're expecting her to rock up to the room herself. That's one of the logistical problems I encountered. I booked the room online, went to the counter to get the keys, then I gotta go to work, I have the other key for her. Now what? Still gotta hang around to meet her at the lobby. Know what I mean?
Hotels I used:
Ibis on Pitt St near the corner of Goulburn St. Used it about 3 years ago. One of the cheapest in the city and was clean, not too old, and roomy enough. Only problem was the heavy foot traffic out the front. I actually bumped into an old acquaintance there while waiting for my girl!
The hotel on top of Capitol Square. Can't remember the name. This one was a real shithole. Used about 2 years ago. Very old, dumpy looking furniture, a bit too close to the street level so you'll have to close all the blinds. The only good thing about it is the rooms are huge because the building is so old.
Holiday Inn near Chinatown. Used last year. It's the one on Harbour St. Reasonably cheap. Big rooms, clean, but furniture older than Ibis. Nice room actually. Only problem was I got stood up by my girl and just used it as a home office the whole day, haha jokes on me!