You should change all GST tax codes from 6% to 0% on 1st June, 2018 onward. Here are some scenarios of the taxable period that may run over: Taxable Period March – May Submission will be in June and the GST return will report all GST values for the 3 months at 6%. Taxable Period April – June Submission will be in July and the GST return will report only GST values for the first 2 months, April & May at 6%; and 0% for June. Taxable Period May – July Submission will be in August and the GST return will report only GST values for the first month, May at 6%; and 0% for Jun & July. Taxable Period June – August Submission will be in September, if it is still required by then but the GST values will all be at 0%.