Pat roast dry with paper towels. Using a sharp knife, score the rind at 1cm intervals all over without piercing the meat. Rest in the fridge uncovered for an hour (better still, overnight).
Just before cooking, place the roast on a wire rack in the sink. Pour a jug of boiling water over the rind. Pat dry thoroughly with paper towel. Rub the roast with 1 tablespoon of oil and half a tablespoon of salt (more if you like salty crackling). Make sure the oil and salt penetrate the scores.
Place the pork on wire rack inside a baking tray and cook at 240°C until the rind crackles, up to 40-50mins.
Turn the oven down to 170°C and cook for 30-35 minutes per kg, depending on how well you like your roast cooked. Once cooked, let the roast rest for 10-20 minutes before slicing.
By letting it dry out in the fridge and also patting it with paper towels before cooking.
When scoring the fat, be careful not to cut through the fat layer into the meat.
Pay particular attention to getting the oil and salt into the score marks. The salt reacting with the fat is what causes crackling to puff up and become crisp.
The perfect New Zealand pork roast is all about time and temperature. The key to crispy, crunchy crackling is getting the rind as dry as possible then an initial high heat sizzle.