After an improvement has been authorized pursuant to section 67.457, the governing body of the city or county may issue temporary notes of the city or county to pay the costs of such improvement in an amount not to exceed the estimated cost of such improvement, and such temporary notes shall be general obligations of the city or county. General obligation bonds of the city or county shall be issued and sold as provided in section 67.455 to refund, retire and pay off such temporary notes and any accrued interest thereon to the date of payment.