OK, a brief discussion on immunity, stolen from a post by me back in 2001: there is “use immunity” and there is “transactional immunity.” A witness asserting the Fifth Amendment may be granted use immunity, which simply means that none of his testimony, nor any evidence derived from his testimony, may be used against him. He may still be prosecuted for the acts he testifies to, however, as long as independent evidence is developed.
Transactional immunity is the complete immunity from prosecution for all transactions covered by the testimony, no matter what the source of the evidence. Use immunity - from a defense lawyer’s point of view - is a cheap substitute, a ploy based on a case called Murphy v. Waterfront Commission. Unfortunately, the courts have upheld, many times, the principle that use immunity is coextensive with Fifth Amendment protection - that is, a grant of use immunity is sufficient to overcome your Fifth Amendment rights and force you to testify.
A person claiming the Fifth Amendment privilege must have a reason to do so, and his bluff may be called, so to speak, by a grant of use immunity. Once he has this immunity, he cannot refuse to testify, and if he then fails to provide any testimony upon which his Fifth Amendment claim might have been based, he may be subject to prosecution for obstruction of justice.
The Fifth Amendment protects against both state and federal criminal actions – in other words, a state judge may grant use immunity to you for your conduct that violated state law, and thus force you to testify…but not if your conduct also violated federal law.
In this case, the claim seems to be that by operation of state law, the witness has use immunity for anything he says to the state legislature. If this is true – and I have no idea what New Jersey law actually says on the subject – then the witness is still entitled to remain silent using his Fifth Amendment privilege if his testimony would subject him to federal criminal liability.