N.J. Stat. Ann. § 54:4-33

No deduction from the assessed value of real property shall be made by the assessor on account of any mortgage debt, but the mortgagor or owner of the property paying the tax on mortgaged real property shall be entitled to credit on the interest payable on the mortgage for so much of the tax as is equal to the tax rate applied to the amount due on the mortgage, except where the parties have otherwise agreed, or where the mortgage is an investment of funds not subject to taxation, or where the parties have lawfully agreed that no deduction shall be made from the taxable value of the lands by reason of the mortgage.