Add stock, water and pork to pot, bring to a boil then cover and simmer for an hour.
Soak watercress in cold water for 10 minutes. (This removes bitterness) Squeeze out moisture and break into pieces. Set aside.
Add kumara, onion, green onion and tomatoes to stock and simmer for 15 minutes.
Remove pork and chop into pieces. Return meat to stock and boil for 5 minutes. Add salt and watercress and simmer for 15 minutes.
Meanwhile make the doughboys. Cut butter into dry ingredients until it resembles coarse cornmeal. Stir in enough milk to make a stiff, slightly sticky dough.
Drop either teaspoon or tablespoon sized amounts of the doughboy mixture into the the boiling pot, cover and cook for about 10-15 minutes. Don't lift lid while cooking. Larger doughboys will take a bit longer.
Serve with a garnish of chopped cilantro.