1.For soup, place all ingredients and 2 litres water in a large saucepan and bring to the boil over medium heat. Add wakame, mushrooms and corn and keep at a gentle simmer.
2.Meanwhile, cook noodles in boiling water for 4 minutes or until al dente, drain and divide among bowls, top with crabmeat, cha siu and green onions, then ladle soup mixture over and serve immediately.