Whenever, in the judgment of the attorney general, the testimony of any witness, or the production of books, papers, or other evidence by any witness, in any case or proceeding before any grand jury or superior court involving any violation of this chapter or any of the offenses enumerated in N.H. Rev. Stat. § 570-A:7, or any conspiracy to violate this chapter or any of the offenses enumerated in N.H. Rev. Stat. § 570-A:7, is necessary to the public interest, the attorney general, or a county attorney upon the written approval of the attorney general, shall make application to the superior court that the witness shall be instructed to testify or produce evidence subject to the provisions of this section, and upon order of the court such witness shall not be excused from testifying or from producing books, papers, or other evidence on the ground that the testimony or evidence required of him may tend to incriminate him or subject him to a penalty or forfeiture. No such witness shall be prosecuted or subjected to any penalty or forfeiture for or on account of any transaction, matter or thing concerning which he is compelled, after having claimed his privilege against self-incrimination, to testify or produce evidence, nor shall testimony so compelled be used as evidence in any criminal proceeding (except in the proceeding described in the next sentence) against him in any court. No witness shall be exempt under this section from prosecution for perjury or contempt committed while giving testimony or producing evidence under compulsion as provided in this section.