Preheat the oven to 180°C, fan 160°C, gas mark 4. Unwrap the lamb and carefully slash the meat 20-30 times with a sharp knife.
Poke small sprigs of the rosemary into the slashes, then place the leg in a roasting tin and drizzle with 1 tablespoon of the olive oil. Season to taste with salt and pepper.
Cover loosely with foil and roast in the preheated oven for 1 hour and 30 minutes, then increase the oven temperature to 220°C, fan 200°C, gas mark 7 ready for the vegetables.
Toss the prepared vegetables with the remaining 3 tablespoons of oil.
Scatter around the lamb and return to the oven to roast for a further 35-45 minutes until the lamb is cooked and the vegetables are tender and lightly charred. Turn the vegetables once during cooking to ensure even browning.
Place the lamb and vegetables on a serving plate. Cover loosely with foil and leave to rest in a warm place.
Spoon off as much fat as possible from the roasting tin, then place the tin on the hob and add the water or lamb stock. Bring to the boil, stirring to scrape up the pan juices. Simmer for 1-2 minutes and season to taste.
Carve the lamb and serve with the vegetables and a little of the pan gravy.
*All information included in this recipe has been provided by New Zealand Lamb.