Excess Concessional contributions are taxed at 31.5% (In addition to ordinary 15% contributions tax) with the amount of your excess contributions also counting towards the Non-concessional limit. Excess Non-concessional contributions are taxed at 46.5%.
Exceeding the contribution limits is not uncommon, particularly as many people don’t realise that their employer superannuation guarantee contributions count towards Concessional contributions limit.