You pay the two different bands on a per subject basis. Basically the 8k they say for law is the max they charge if you are doing all law subjects for example so usually I pay around $1k to $1.5k per law subject and $300-$900 for an arts subject depending on the percentage it is of a full study load.
Ahh here I can give you an example, keep in mind its a bit less than usual as I am doing slightly less than a full load because I overloaded twice the year before. See how the arts one is band one and the other two are band three but they dont hit you for the whole band for both.
09-Jan-2008 Fee 2007 HECS Band 1 SOC 365 01-Aug-2008 $899.00 $899.00
09-Jan-2008 Fee 2007 HECS Band 3 LAW 315 01-Aug-2008 $1,124.00 $1,124.00
09-Jan-2008 Fee 2007 HECS Band 3 LAW 317 01-Aug-2008 $1,124.00 $1,124.00
Generally I pay anywhere up to about $8-10k per year for Arts/Law but those doing comm/law will pay a bit more because Comm is band two and arts is band one. Next year it will be likely to be more because I will be doing only law subjects.