Hey Doggie Diva,
Might be able to help you out here. Obviously you are talking about Sydney. I've stayed at a good number of the hotels in Sydney CBD, or at least the bigger ones.
What's your budget for a room? How many people? Why is the no party rule an issue?
If it's getting ready for a night on the cans, then there are plenty. If it's to literally hold a party in the city, good luck.
Now the reason (as far as I understand it) as to why you can't actually hold a party in most hotel rooms is fire safety standards. The hotel needs to know who is checked/booked in, and the maximum number of people they could potentially have in one room.
Sounds like you have a decent budget by the places you are talking about. Generally though, one you exceed the total pax of the room, then you'll need to rent another.
What you can do if you are thinking of having a party, is book rooms at a place like the Shangri-La, and then head either to Level 36, or to the lobby lounge for drinks and stuff with friends.
Can help out more on your reply.