I have always been lucky booking last minute accommodation through www.lastminute.com.au and also www.agoda.com
I chose a hotel recently in Melbourne through Lastminute by booking their Mystery Hotel for $150 and was surprised to booked into a 5 star hotel close to everything! They called it the Paris end, I still cant work that one out.
Another one I sometimes I stay at is Mantra Apartments - same price. almost 6 years ago
You can also go to : What If : it gives you daily cheaper rates and from 2 star onwards. Good luck. Make sure you have great walking shoes ,,,so much to see and very spread out. Casino,Victoria market, aquarium, zoo, great food and so many little lane ways full of funky shops. almost 6 years ago