The city shall not sell, exchange, lease, or in any way dispose of any property, easement, equipment, privilege, or asset needed to continue the operation of any municipal public utility, unless the proposition to do so is approved by at least three-fifths of the electors of the city voting on the question at a regular or special city election. All contracts, grants, leases, or other forms of transfer in violation of this section shall be void and of no effect as against the city. The restrictions of this section shall not apply to the sale or exchange of articles of machinery or equipment of any municipally owned public utility, which are no longer useful or which are replaced by new machinery or equipment, or to the leasing of property not necessary for the operation of the utility, or to the exchange of property or easements for other needed property or interests in property.