In nj, how far can the ncp fall behind on child support before the court does anything about it? And what does the court normally do to enforce it?

The court will do nothing until and unless the arrears are brought to is attention.

Most states (and I believe NJ is one) have Child support collection agencies. Some are mandatory, some aren't. Have you visited this site:
NJ Child Support - Welcome

Seems to be a central place for you to get help in collecting support.

In our great state of NJ child support collection and enforcement is handled by the Dept. of Probation. And they will be on you like bees on honey the minute you fall behind even a few hundred dollars. They will not hesitate to haul you in to court in front of a hearing officer to explain why you've fallen behind and to make arrangements for paying your arrears as well as what's currently due. They'll also garnish your wages, intercept your tax refund, place a lien on your house and anything else to get the money out of you.