is the 30000 GST threshold based on my Canadian sales only

No, and this one is expensive to get wrong.

The small-supplier test is based on your worldwide taxable supplies, not your Canadian sales alone. Sales to customers in the United States and elsewhere count toward the same $30,000 figure.

The CRA measures it over four consecutive calendar quarters or within a single calendar quarter, and the calculation is on total revenue before expenses, from worldwide taxable supplies including zero-rated ones. Revenues of associated persons count too. Some things are left out of the calculation: financial services, goodwill, and sales of capital property among them.

Why online sellers get caught by this

If you sell on Shopify to both Canadian and US customers, it's natural to look at your Canadian revenue and conclude you're under the line. But an exporting seller can be well past $30,000 worldwide while their Canadian sales look modest: and crossing the threshold has consequences that start on the day you cross it, not at year end.

Note that exporting doesn't remove a sale from the count. Zero-rated sales are still taxable supplies; they're taxed at 0%, which is not the same as being outside the system.

What we'd suggest

Look at your total worldwide sales, not the Canadian subtotal, and if you're anywhere near the line, get your accountant to confirm where you actually stand and from what date. Talisk shows you the number across every channel you've connected; whether it means you need to register is your accountant's call.