I plan to retrench more than 15 employees who are not union members or covered by an award or enterprise agreement. Do I have an obligation to consult with employees?

No, but the Fair Work Commission may create that obligation. A union with relevant coverage over the employees being retrenched, or the employees themselves, can apply to FWC for orders requiring that consultation (section 531 Fair Work Act). The union can make this application even if it does not have any actual members affected by the retrenchments.