Spend 4 weekends in the area pretending that you want to rent a place identical to what you are buying. Go talk to property managers, read the local paper, look in shop windows. You will figure out pretty fast what the real rent is.
Make sure you are accounting for ALL costs like body corporate fees. And that the rent hasnt been artificially inflated.
www.streetsales.com.au
www.onthehouse.com.au
www.myrpdata.com.au
www.pdslive.com.au
Some are free some arent.