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So if I paid for the insurance before GST registration, but then canceled and received a refund on June 2023 (after GST registration), I have to pay back GST to ATO, even though I didn't claim GST credits?
You wouldn't need to pay us GST if you got a refund. You would need to apply GST to your sales for the period you're registered. For example if you backdated your registration to 1/02/2023 you'll have to pay us GST if you made taxable sales since then.