The state treasurer may, with the approval of the state board of finance, issue bonds in such form as the state treasurer shall determine to be designated refunding bonds for the purpose of refunding any of the bonded indebtedness of the state now existing or hereafter created which has or may hereafter become due and payable at the option of the state or by consent of the bondholders or by any lawful means and for the payment or redemption of which there are insufficient funds available in the state treasury.