Yes, you do pay 15% tax on money that you salary sacrifice into your super (or 30% if you earn more than $250,000). But because you're taking that money out of your before-tax pay, you don't then pay your normal tax rate as well.
If you made after-tax contributions to your super instead (also called personal contributions or voluntary contributions), you wouldn't pay any more tax unless you claim a tax deduction for it. If you do claim a tax deduction, you would pay both your normal tax rate on your salary and the 15% super tax as well.