In Australia, you must register for GST if your business has a GST turnover (gross income minus GST) of $75,000 or more per year, or $150,000 for non-profit organisations. Taxi and ride-sharing drivers must register regardless of turnover.
GST turnover is your gross business income, not profit. If you're approaching the threshold, you must register within 21 days of exceeding it. The ATO can backdate your GST obligations if you should have been registered but weren't — meaning you'd owe GST on all sales made since you crossed the threshold, without having collected it from customers.
You can also register voluntarily even if you're under the threshold. This lets you claim GST credits on business purchases and can be advantageous if your customers are GST-registered businesses (they can claim credits on what they pay you). However, voluntary registration means you must charge GST on all taxable sales and lodge BAS returns, which adds compliance burden.
Once registered, you add 10% GST to your taxable sales, collect it from customers, and remit the net amount (GST collected minus GST credits on purchases) to the ATO via your BAS. You must issue tax invoices for sales of $82.50 or more (including GST).