Wash rice under cold water and place into a saucepan with water. Bring to the boil, reduce heat to medium and simmer for 5 minutes. Cover and cook over very low heat for 15 minutes, then remove from the stove. Rest covered for 10 minutes.
Place vinegar, mirin, sugar and salt into a small saucepan. Cook over a low heat, stirring for 3 minutes or until sugar has dissolved. Refrigerate until cold.
Spread rice over a large tray. Drizzle vinegar mixture over rice. Using a wooden spoon, make cutting strokes into rice to distribute vinegar mixture. Do not stir.
Place a sheet of nori shiny side down onto a bamboo mat. Place 3/4 cup of rice mixture onto nori. With wet hands, gently spread rice evenly over nori, leaving a 3cm border at the far end. Place a strip of cucumber onto rice, 3cm in from the edge. Place an avocado slice and a seafood stick next to the cucumber. Roll up. Repeat with remaining nori and fillings. Cut each roll into 6 pieces.