Unless required for disposition of an ex parte matter authorized by law, a member or employee of an agency assigned to make a decision or to make findings of fact and conclusions of law in a contested case shall not communicate, directly or indirectly, in connection with any issue of fact, with any person or party, nor, in connection with any issue of law, with any party or his representative, except on notice and opportunity for all parties to participate. This prohibition begins at the time of the notice of hearing. An agency member may communicate with other members of the agency and may have the aid and advice of the agency staff other than the staff which has been or is engaged in investigating or prosecuting functions in connection with the case under consideration or a factually related case. This section does not apply to an agency employee, or party representative with professional training in accounting, actuarial science, economics, financial analysis or rate-making, in a contested case before the financial institutions bureau, the insurance bureau or the public service commission insofar as the case involves rate-making or financial practices or conditions.