My guess is that activated carbon is the wrong type of filter for your water problem. AC is great for organics but will not remove other impurities such as hard water - including sulphates. For those you need ion exchange or reverse osmosis. The advice I've read is to first determine the cause of your water quality problem and then buy the appropriate filter. Unfortunately a quick google didn't find any good info on qld groundwater.
http://www.lenntech.com/sulfates.htm
http://www.msue.msu.edu/objects/cont...01500610.html/