Yes, you do pay 15% tax on money that you salary sacrifice into your super (and an extra 15% tax may apply if your income plus before-tax contributions are over $250,000/year). 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.