From the looks of it, they embedded tater tots into the pizza's crust prior to baking. It was probably easy to execute as Hash Bites (their name for tater tots) are regularly offered as a side on the Pizza Hut New Zealand menu.
The pizza crust was introduced about two months ago and cost an additional $3 NZ (~$1.95 US) on a large pizza. The price was just a little less than ordering Hash Bites as a side. So it's basically a choice between getting tater tots with your pizza or on your pizza.
Following the Hash Bites Crust Pizza, they recently brought back the Cheeseburger Crust Pizza in New Zealand:
Photos via Pizza Hut New Zealand.