When researching prices in particular areas I like to purchase the Home price guide postcode report. Cost about $99 and is good for 3 months. I choose the 2 year report and this tells me all properties sold in the previous 2 years and how much they sold for and usually number of bedrooms and landsize.
When I actually find a particular place I want to offer on I purchase a Home price guide property report $55 or something. This report gives me a sales history of the property, and also gives a price range of what the property is worth compared to previous sales. This report also includes previous sales in the area.
I sometimes also purchase a RP data report as well. RP data tends to give a bit more info and has a photo included. If the property is over $400k I tend to purchase both.
I check Home Price guide's free suburb snapshot and RP Data's free suburb profile regularly to keep an eye on the areas I invest in. Both these reports allow me to keep an eye on monthly sales, and prices.
I find these reports invaluable to my research. There are probably other sites as well, I haven't used any others yet, but thats not to say these ones are the best.
Sunshine
