24HRSALE
The ParkRoyal Hotel in Darling Harbour is having a 24-hr sale for stays before the end of May. Book online (see link) or phone 1800 192 144 and quote '24HRSALE'.
I stayed here for a conference a couple of months ago - the rooms are very good, and the breakfast is great (but damned expensive so hopefully you can get work to pay for it!). The internet charges for rooms are ridiculously high but if you join their GHA Discovery program (free, just sign up either when you arrive or depart) they'll waive internet charges.
A quick check of the same room one week after this special period shows a best price of $200/night.
Very easy to walk to Darling Harbour, Chinatown, CBD, etc.
EDIT FOR T&Cs: Valid for bookings made on the 25th May 2011 for stays between 25th May till 31st May 2011. Rate inclusive of room only. Full prepayment required at the time of booking, non-refundable, no cancellation and subject to availability.