I've set it up, and it works even though my iPhone's case (note that your phone does need to be unlocked).
Watch Michelle's video below for how to set it up - and make frequent use of pause - or use this step-by-step guide from our friends at Tech Advisor (if you get stuck right at the start, pull down from the middle of your iPhone's screen to make the search bar appear, then type "Shortcuts").
Yes, it's a multistep process, but it only takes a couple of minutes to set up - and then you'll be scanning QRs in a couple of seconds.
Such "back tap" shortcuts work with any model from the iPhone 8 onwards, as long as it's running iOS 14, click Settings > General > Software Update.
If you haven't already, it's a good idea to upgrade to iOS 14 - the latest version of the free software that runs your iPhone - because you need it to enable the NZ Covid Tracer app's new Bluetooth feature (which tracks who you've been in contact with, while posters record where you've been; it's important to use both features).
Apple lets you use the new back tap to set up two short cuts - one a double-tap, the other a triple tap.
So once you've set up your NZ Covid Tracer back tap shortcut, you can follow the same steps to set up a one for another regular function, like taking a photo or a screengrab.