Place apples and water in a large saucepan. Bring to the boil; reduce heat. Simmer for 25 minutes or until apples are tender, stirring occasionally.
Wrap mustard seed in muslin or cheesecloth; add to apples. Add tomatoes, onions, garlic, sultanas, sugar, curry powder, cayenne pepper, salt and vinegar to the apples. Stir until sugar has dissolved.
Bring the mixture to the boil. Reduce heat; simmer for 3 hours, stirring occassionally, until mixture is thick and pulpy. Remove and discard wrapped mustard seed. Seal chutney in hot sterile containers. Store in refrigerator for up to 6 months.