Streets of Laredo are made up of brothers Dan and Dave Gibson, Dave's wife Sarah Gibson and Si Moore.
Dan is the former frontman for pop-punk band Kingston, while Dave sang for Elemenop.
Judges hailed them as another stand out group from a country "brimming" with successful musical acts.
"In a country brimming with talent, Streets of Laredo stand out for their indie folk sound that is completely current yet timeless.
"Their winning song, Girlfriend is infectious, catchy, and creative, and they might just be the next big thing to come out of New Zealand, following in the footsteps of Kimbra, Gin Wigmore, and Lorde."
Wes Mack of Calgary, Canada took out the Unsigned Only grant prize for his song Duet.
Skipping Girl Vinegar from Melbourne won second prize in the AAA category for Chase The Sun.
Streets of Laredo recently recorded their debut album in New Zealand.
Watch their video for Girlfriend.