The governor and council may authorize the issuance of refunding bonds in order to pay, at maturity or upon earlier redemption or acceleration, all or part of any issue of bonds then outstanding that were issued by the state or with a direct state guarantee; provided, however, that unless the governor and council specifically provide otherwise no such bonds shall be issued unless the treasurer determines that the present value, discounted at such rate as the treasurer deems appropriate, of the principal and interest payments on the refunding bonds is less than the present value, discounted at the same rate, of the principal and interest payments on the bonds to be refunded. The proceeds of such refunding bonds may be used to pay the principal of the refunded bonds, any redemption premium thereon, all or part of the interest coming due on or prior to the date or dates on which the refunded bonds are paid, and the costs of issuing and marketing the refunding bonds. The issue of refunding bonds shall be subject to the same requirements and provisions of law as would then be applicable to the issue of the bonds being refunded, except as provided in this section. The proceeds of refunding bonds, exclusive of any amounts used to pay costs of issuing and marketing the refunding bonds, shall be held in a separate fund and in trust until they are applied to pay bonds. While such proceeds are held in trust they may be invested in accordance with N.H. Rev. Stat. § 6:7 and N.H. Rev. Stat. § 6:8 and the income derived from such investment may be expended by the treasurer to pay the principal of, redemption premium if any, and interest on the refunded bonds until they are paid.