Place rice and 2 cups of cold water in a medium saucepan. Bring to the boil over medium heat. Reduce heat to low. Simmer, covered, for 15 minutes or until rice is just tender. Remove pan from the heat. Set aside, covered, for 10 minutes.
Add sushi seasoning to rice. Using a fork, gently fold through rice. Spread rice mixture over a large tray lined with baking paper. Set aside for 1 hour or until rice has cooled.
Mash avocado and lemon juice together in a bowl. Using damp hands, divide rice into 16 equal portions. Place on a large board lined with baking paper. Using fingertips, press each rice portion into a 1cm-thick round.
Spoon 1 teaspoon of avocado mixture in the centre of each round. Divide carrot, cucumber and salmon between rounds. Fold rice mixture around filling to form a ball. Roll balls in sesame seeds. Place on a large tray. Cover and refrigerate for 1 hour. Serve with soy sauce.