N.H. Rev. Stat. Ann. § 161-C:23

Interest at the rate specified in RSA 336:1 on any support debt due and owing to the department under RSA 161-C:4 may be collected by the commissioner. No provision of this chapter shall be construed to require the commissioner to maintain interest balance due accounts, and said interest may be waived by the commissioner, if said waiver would facilitate the collection of the debt.