The department shall perform an examination of each credit union authorized to do business under the Act at least once during each 12-month period. Annual examination intervals may be extended by the Department to a maximum 18 months, subject to safety and soundness considerations. Intervals between examinations shall not exceed 18 months, unless a longer interval is authorized in writing by the commission. In lieu of conducting an examination required by this rule, the commissioner in the exercise of discretion may accept examinations or reports from other credit union supervisory agencies or insuring organizations.