“Agency” means a board, bureau, commission, department, division, office, officer, or other administrative unit, including the agency head, and one or more members of the agency head or agency employees or other persons directly or indirectly purporting to act on behalf of or under the authority of the agency head. To the extent it purports to exercise authority pursuant to this chapter, an administrative unit otherwise qualifying as an agency shall be treated as a separate agency even if the unit

is located within or subordinate to another agency.