(a) Upon a complaint in writing made against any natural gas public utility governed by this act that any rates or rules and regulations of such natural gas public utility are in any respect unreasonable, unfair, unjust, unjustly discriminatory or unduly preferential, or both, or that any rule and regulation, practice or act whatsoever affecting or relating to any service performed or to be performed by such natural gas public utility for the public, is in any respect unreasonable, unfair, unjust, unreasonably inefficient or insufficient, unjustly discriminatory or unduly preferential, or that any service performed or to be performed by such natural gas public utility for the public is unreasonably inadequate, inefficient, unduly insufficient or cannot be obtained, the commission may proceed, with or without notice, to make such investigation as it deems necessary. No order changing such rates, rules and regulations, practices or acts complained of shall be made or entered by the commission without a formal public hearing in accordance with the provisions of the Kansas administrative procedure act, of which due notice shall be given by the commission to such natural gas public utility or to such complainant or complainants, if any. Any public investigation or hearing which the commission shall have power to make or to hold may be made or held before any one or more commissioners. All investigations, hearings, decisions and orders made by a commissioner shall be deemed the investigations, hearings, decisions and orders of the commission, when approved by the commission.