When a SEZ unit or SEZ developer procures any goods or services from an unregistered supplier, whether the SEZ unit or SEZ developer needs to pay IGST under reverse charge or these will be zero rated supplies

Supplies to SEZ unit or SEZ developer have been accorded
the status of inter-State supplies under the IGST Act.
Under the GST Law, any supplier making inter-State supplies
has to compulsorily get registered under GST. Thus anyone
making a supply to a SEZ unit or SEZ developer has to necessarily
obtain GST registration.

(FAQ 6: EXPORTS)