Any portion of the principal proceeds derived from the sale of the bonds which the board of directors of the authority may determine is not then needed for any of the purposes for which the bonds are authorized to be issued shall, on order of the authority, be invested by the State Treasurer in any securities which are direct and general obligations of the United States of America or the principal of and interest on which are unconditionally and irrevocably guaranteed by the United States of America. Any such securities may, at any time and from time to time on order of the authority, be sold or otherwise converted by the State Treasurer into cash. The income derived from any such investments shall be disbursed on order of the authority for any purpose for which it may lawfully expend funds.