N.J. Stat. § 54:49-3
Any taxpayer who shall fail to pay any State tax on or before the day when the same shall be required by law to be paid shall pay in addition to the tax, unless otherwise provided in the law imposing such tax, interest and penalty, if any, on said tax at the rate of three percentage points above the prime rate assessed for each month or fraction thereof, compounded annually at the end of each year, from the date the tax was originally due until the date of actual payment. If the director is empowered by the law imposing such tax to grant an extension of time in which the tax shall be paid, the taxpayer shall be liable for the payment of interest on the unpaid tax at the rate of three percentage points above the prime rate assessed for each month or fraction thereof, compounded annually at the end of each year, from the date that such tax was originally due to the date of actual payment. Any amount of tax unpaid after the time when the same shall be required by law to be paid, or after an extension of time if granted, is an underpayment for the purposes of R.S. 54:49-4.
N.J.S. § 54:49-3