N.J. Stat. § 45:21-7
Any policy filed under the provisions of this chapter may be canceled, as to accidents occurring subsequent to the effective date of such cancellation, by the insurer or by the owner upon the terms and in the manner therein provided for; but such cancellation shall not be effective unless and until twenty days' notice in writing of such cancellation shall have been given said clerk. Service of such notice upon said clerk may be made either personally or by mail, and if by mail shall be deemed complete when the notice is deposited in the post office, registered, postage prepaid, and directed to said clerk at his official address. Immediately upon receipt of notice by said clerk of the cancellation of any such policy, or if the authority of the insurer to transact such business in this state shall be revoked, he shall require the owner to replace such policy with another good and sufficient policy in accordance with the provisions of this chapter, in default of which the owner's certificate shall be revoked for all motor vehicles covered by such policy, and the owner shall thereupon cease to operate, or cause or permit to be operated, any such motor vehicle. Upon revocation of the certificate of any owner by said clerk, other than for failure to file another policy upon notice of cancellation, as herein above provided for, notice shall be sent by said clerk to the insurer, and such notice of revocation shall operate automatically to cancel such policy for the motor vehicle for which the certificate was revoked, as to all accidents occurring subsequent to the revocation of the certificate, and such insurer shall be liable for the return of the unearned premium upon such policy, as in case of cancellation by the insurer.
N.J.S. § 45:21-7