I observed the exact same mileage (11.8km/l) on another recently-purchased brand new Yaris CVT 1.3L driven less than 1500km, so it's possible that this might just be the mileage the car gets.
Reset your fuel consumption indicator (average consumption, not instantaneous) when you fill up the tank. Then, drive normally for a few days without resetting it. The number will vary greatly for the first few minutes or hours of total driving, but will stabilize. That is your average fuel consumption. You can also drive until your next fill-up and verify your mileage by dividing the number of kilometers driven by the number of liters consumed. The values should be the same or similar.
Let us know what happens after you try this. If you're not happy with the mileage, try a more gentle driving technique. Don't use the manual shifting on the CVT, use a light foot, and coast gently to a stop instead of using the brakes all the time.
