Provisional assessment: permits provisional GST payment pending final valuation, subject to bond and interest obligations. Provisional assessment allows a taxable person unable to determine value or rate to request provisional tax payment; the proper officer must rule within ninety days and may require a prescribed bond with surety to secure any difference between provisional and final tax. Final assessment must be completed within six months, subject to authorised extensions, and interest is payable on unpaid provisional tax from the day after the original due date until actual payment; refunds consequential to final assessment attract interest under the refund provisions.