Tax is payable at your marginal tax rate on your 'net capital gain'.
The CGT discount of 50% (in the case of most individuals) reduces the amount of your discountable capital gains to be included in your taxable income (after the capital losses and other concessions not likrly to affect most investors here).
i.e. if you sold a share for $100 that you had owned for more than one year that had cost you $50, the gross gain of $50 would be reduced by 50% and the remaing $25 would be included in your taxable income and you would pay tax on that $25 at your marginal tax rate (bracket).
It is easy to get tax wrong and as always I recommend seeing an accountant to look after your tax affairs
