This item is second hand but the condition in great condition.
 
This dress is the Flutter Tee Dress by the New Zealand streetwear brand Thing Thing. 
 
Design Details
Style: A casual mini-length tee dress with a relaxed fit.
Sleeves: Features long, volumed bell (or "flutter") sleeves that gathered at the mid-arm to create a flared silhouette.
Fabric: Made from a soft, stretchy ribbed knit material.
Composition: Typically 80% Polyester, 15% Rayon, and 5% Spandex.
Neckline: Simple round/crew neckline