N.J. Stat. § 43:16A-3.13
For municipalities in which the provisions of P.L. 1944, c.255 (C.43:16A-1 et seq.) took effect on or after January 1, 1992 and prior to the effective date of this supplementary act as a result of a referendum in the municipality or the adoption of an ordinance or resolution by the governing body of the municipality as provided by law, the State shall be liable, beginning with the effective date of this act and continuing during their membership in PFRS, for the difference in the normal contribution between what the employer would pay under PERS and what the employer would pay under PFRS for the policemen and firemen in service prior to the date the provisions of P.L. 1944, c.255 took effect in each such municipality who elected to become members of PFRS pursuant to section 13 of P.L. 1945, c.148 (C.43:16A-29). The municipality, however, shall be liable for employer contributions to PFRS for any policeman or fireman whose service began or begins on or after the date those provisions took effect.
N.J.S. § 43:16A-3.13