You are not obligated to provide your TFN to your superannuation fund. However, if you do not provide your TFN, your fund may be taxed at the highest marginal tax rate plus the Medicare levy on contributions made to your account in the year, compared to the concessional tax rate of 15%. Your fund will usually deduct this additional tax from your account. If your superannuation fund does not have your TFN, you will not be able to make personal contributions to your superannuation account. Choosing to quote your TFN will also make it easier to keep track of your superannuation in the future.