If you exceed your concessional contributions cap, the excess concessional contributions (ECC) are included in your assessable income.
The concessional contributions cap is the maximum amount of before-tax contributions you can contribute to your super each year without contributions being subject to extra tax. From 1 July 2024, the concessional contributions cap is $30,000. inthe event that you exceed your Concessional Contributions Cap, Any ECC not released from your super fund counts towards your non-concessional contributions cap.
ECCs are taxed at your marginal tax rate less a 15% tax offset to account for the contributions tax already paid by your super fund. That is, the amount of tax on the excess amount is reduced by 15%.
Source: ATO