I have done a quick search and the official guidance from ABF is as follows:
You can remove your goods from the packaging and use the goods before you depart. You can choose to leave the container/packaging behind as long as the officer can verify the goods against the corresponding tax invoice when the TRS claim is processed.
What I'm unsure about is the part I have bolded above. What exactly do they mean by verifying the goods against the tax invoice? Do they need to see the serial number/model number on the box?
The items I'm looking to claim are an Apple Watch Series 9 and a Fitbit Versa 4 if it makes any difference.
On my iPhone they checked the serial number in the settings of the device. No box.