Shopify native Google Ads integration and GTM both firing purchase events
This is the single most common tracking error in Australian e-commerce accounts. Shopify's Google channel integration fires a purchase conversion directly to Google Ads. If GTM is also installed with its own purchase tag — which most accounts set up for additional tracking control — both fire on the same order confirmation page. The result is a purchase conversion count that is consistently twice the actual order volume, and a ROAS figure that Smart Bidding treats as accurate.