Whenever the board deems it necessary, it may issue bonds to refund all outstanding, unmatured bonds of the authority which are subject to call and redemption prior to maturity. The amount of such bonds shall include the principal and, if necessary, the interest of the bonds to be refunded, the premiums payable upon call and redemption thereof and all expenses of the trustee for the refunding, the calling of said bonds for redemption and any other incidental expenses. The refunding bonds shall be authorized and issued in the manner provided for herein as a separate issue or as a part of another issue for any purpose authorized in this part.