Heat the olive oil in a saucepan over medium heat. Stir in the onion and garlic, cook until tender. Mix in zucchini and coat in the olive oil. Pour in tomato puree and water. Season with oregano, basil and chilli. Dissolve sugar in the sauce. Reduce heat to low, and simmer 1 hour, stirring occasionally.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
In a large bowl, mix sauce and cooked past shells. Top with cheese to serve.