Unless required for the disposition of ex parte matters authorized by law, members or employees of the department assigned to render a decision or to make findings of fact and conclusions of law in an adjudicatory proceeding 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 upon notice and opportunity for all parties to participate. Members and employees of the department may communicate with other members and employees of the department, and may have the advice of department staff other than staff which has been or is engaged in the investigative or prosecuting functions in connection with the case under consideration or factually related cases.