It depends on what on extra is and for instance the superannuation contribution will be taxed separately.

If you have a read of the personal taxation information on Australian Taxation Office Homepage that'll give you an idea of what you pay tax on and what you will not and what kind of things you can claim as deductions etc.

But usually, something that is shown as part of a package but not actually salary is classed as a benefit and the employer possibly pays fringe benefits taxation on it.