Here's where I thimk the borders are, in my simplest way. So if I had to categorize any one county in NJ here's how it would be:
North: Sussex, Passaic, Bergen, Warren, Morris, Essex, Hudson

Central: Hunterdon, Somerset, Union, Middlesex, Mercer, Monmouth

South: the rest :3

I traced off a map, some of it is cut off but that's still fine.