N.J. Stat. Ann. § 40:41A-63

The office of county supervisor shall be deemed vacant if the incumbent moves his residence from the county or he is by death, physical or mental illness or other casualty unable to continue to serve as county supervisor. Any vacancy in the office of county supervisor shall be filled in the manner prescribed by law for the election of county officers at the next general election occurring not less than 60 days after the occurrence of the vacancy. The board of freeholders shall appoint one of their number to serve as acting county supervisor until a successor has been elected. During the temporary absence or temporary disability of the county supervisor the administrative officer shall serve as acting county supervisor, except that he shall not preside over freeholder board meetings.