A calculator of this nature can only be as accurate as the data you input, and the functions and formulae used to deliver an output (i.e. the ATAR). The data you input in those ATAR calculators is your HSC mark, but in reality, UAC uses the average of your raw HSC exam mark and your moderated assessment mark. The formulae used in ATAR calculators uses the UAC tables, whether just for the previous year, or an average or some distribution of several previous years. They also need to interpolate the data since UAC only gives a few data points to work with. In reality, scaling is carried out afresh each year, so last year's scaling does not apply to this year's scaling. They also have the full range of data to work with and they are the ATAR calculators.
Apart from UAC themselves, there are no accurate ATAR calculators. There are calculators that use the data from last year, so if I find an ATAR calculator that uses the 2011 UAC tables, and I input my HSC marks, it will be quite close because it uses the data that I was a part of (because I was in the 2011 cohort).
A simple Google search will yield the most popular ATAR calculators.