The union has complained to the Office of the Ombudsman.
''Its non-response suggests it has something to hide,'' Association of Salaried Medical Specialists executive director Ian Powell said.
After fierce resistance from staff about its proposal to outsource the service, the board put off the decision, which is now expected early next month.
Mr Powell said in a statement managers were marginalising and ignoring their own clinical staff.
''Unfortunately, the DHB's management does not appear to appreciate the value of what it has. It seems hellbent on ditching the service in an attempt to save money, based on a proposal that is seriously flawed, containing erroneous information and understandings.''
After receiving no response to an Official Information Act request for the documents, the union has appealed to the Office of the Ombudsman.
Southern DHB women's, children's, and public health medical director Dr Marion Poore said in a statement the board was working through a thorough process to ensure it provided the best possible service.
''We have no concerns about the quality of the service, or its staff, and are proud of the service and the way it functions.
''However, it is a small service and we simply want to ensure it is clinically and financially sustainable for the future.
''As a DHB, we are working very hard to ensure positive relationships with our clinical staff and will continue to actively work with [the union] on how we can do this.''
