(a) In the performance of his duties and in relation to any investigation or inquiry which the commissioner is authorized to conduct, the commissioner or any agent designated by him in writing may:

(1) Administer oaths and take affidavits;

(2) Conduct hearings;

(3) Examine witnesses under oath; and

(4) Subpoena the attendance of witnesses and require the production of books, papers, records, and documents and, subject to the rights of the taxpayer as to rights of privacy guaranteed to him by the Constitution and laws of this state, may examine such items and the books, records, inventories, or business of any taxpayer or of any fiduciary, bailee, or other person having knowledge of the tax liability of any taxpayer or knowledge pertinent to the investigation or inquiry.

(b) The powers conferred pursuant to subsection (a) of this Code section shall be exercised with due regard to the rights of the citizen and, when invoked, subject to the approval of the superior courts of this state.

(c) The commissioner, pursuant to his duties in relation to the collection of state ad valorem taxes, shall investigate settlements by tax collectors or tax commissioners and take appropriate action to collect any revenue due the state which has not been collected or, having been collected, has not been paid to the commissioner. No official or person may employ or commission any person to collect any of such taxes on a commission basis.