The super contributions your employer makes from your before-tax income (concessional contributions) are taxed at 15% by the Australian Taxation Office. If you exceed the concessional contributions cap of $25,000 per financial year, the excess will be taxed at the top marginal tax rate.
This tax component is displayed in your transaction history as 'Contributions Tax', and is sent to the ATO by Future Super regularly.
The super contributions you make after tax (non-concessional) are not subject to tax unless you exceed the cap of $100,000 in a financial year. If you exceed the cap the excess will be taxed at the top marginal tax rate.
For more information on how super contributions are taxed please see the ATO's website.