In a small bowl, combine the sugar, salt and rice vinegar until dissolved. Set aside.
Place rice in a large saucepan and cover with cold water. Gently massage the rice with your fingers until the water is milky. Pour off the water and refill with cold water - repeat this 3 or 4 times to remove excess starch.
Drain the rice well and place in a saucepan. Add the kelp and 700ml water. Bring to the boil, cover, reduce heat to low and simmer 15 minutes. Remove from heat and let stand five minutes before uncovering.
Spread rice on a large tray, discarding the kelp. Sprinkle over the vinegar mixture and mix well with a flat spatula. Let cool before using in your sushi recipe.