An official PowerShell module for automating Nessus via its API. Comparison: Essentials vs. Professional Nessus Essentials (Free) Nessus Professional IP Limit Up to 16 IPs Plugin Updates Compliance Checks Full (PCI, HIPAA, etc.) Commercial Use Prohibited
If you need unlimited scanning without the cost, (Open Vulnerability Assessment Scanner) is the industry standard open-source alternative. Nessus Cracked Github