When purchasing goods or services from an unregistered supplier, which is considered a reverse charge, self-invoicing is required. As your supplier will not be able to send you a GST-compliant invoice, this puts the responsibility on you in paying the taxes on their behalf. Due to this reason, self-invoicing is required.
By section 31(3)(g), a recipient who is required to pay tax under sections 9(3) or 9(4) must issue a payment voucher when paying the supplier.