Should an individual exceed their concessional contribution cap, the excess contributions are included in their income tax return and taxed at the marginal rate of the individual. (Note: An ECC charge is also applied.)
Should an individual exceed their non-concessional contributions cap (including making use of the bring forward rules, if applicable) the excess amount is taxed at the maximum marginal individual tax rate – inclusive of Medicare levy and temporary budget repair levy, if applicable. (Currently this is 49% for the 2015 year and later)
Note: Unless you choose to withdraw your excess concessional contribution from your superannuation fund, the excess also becomes a non-concessional contribution. If this new non-concessional contribution takes you over the non-concessional contribution cap, your contribution could be taxed twice. To avoid this – be aware of the contribution caps and how much you are contributing to superannuation.