An overpayment of tax happens when you have paid more tax than you were liable to pay. If you have overpaid tax you will get a tax refund. You must claim a tax refund within 4 years of the end of the year in which the overpayment arose or you will not get a refund.
An underpayment of tax is when you have paid less tax than you were liable to pay. You may not know that you have paid too little tax, at anytime tax refunds we review your taxes paid for the previous 4 years and post your refund directly to you.