Duncan, whose debut feature The Red House screened in the New Zealand International Film Festival, received $2,000 cash plus other prizes.
Her winning short film is now eligible to be entered into contention for the 2017 Academy Award short film categories.
Now in its tenth year, the festival is running programmes of its 46 short films in 22 New Zealand cinemas in the coming weeks.
SHOW ME SHORTS FILM FESTIVAL 2015 AWARD WINNERS:
Lightbox Best Film Award: Alyx Duncan, The Tide Keeper
Final Draft Best Screenplay Award: Ryan Heron & Guy Montgomery, Return
DEGNZ Best Editor Award: James Cunningham, Accidents, Blunders and Calamities
StarNow Best Actor Award: Tina Cleary, Cub
Best International Film Award: Jonathan Stein, Out of the Village
DEGNZ Best Director Award: Alyx Duncan, The Tide Keeper
Panavision Best Cinematographer Award: Chris Pryor & Ben Montgomery, The Tide Keeper
NZFC Special Jury Prize: Mhairead Connor, UFO
Best Music Video: Thunderlips, The Eversons - Emily
Best Student Film: Kyan Krumdieck and Annabelle Dick, The Grind
People's Choice Award: Announced in December when audience votes are counted.