If you can find sign up bonuses, SPG is an option in Melbourne or Sydney, where they will always have points nights available whilst they are still selling rooms. Cash + points can be cost effective but harder to come by. I also recommend Accor because of the number of properties that they have. Because you are coming in winter (so particularly for Melbourne), be aware there are usually room sales a few months out. It's a bit of a risk, especially if there is a conference or event on, but I got rooms in the centre of Melbourne over a weekend in June for AUD$150 per night and even the high end properties of each brand were selling for around AUD$300. So you will probably get best value by spending points on Sydney hotels at that time of year.