To the untrained eye, the Duchess of Cambridge was wearing a pleasant but nondescript simple blue and white hooped top to go sailing in Waitemata Harbour.
But the $A86 garment has now sold out after royal fashion followers flocked to copy Kate's nautical look.
Maker Me+Em was inundated with buyers after she wore its "Breton top cobalt/white" while aboard an America's Cup yacht off Auckland during her tour Down Under with husband William.
The top is part of a look popular with the 32-year-old Duchess, who wore an almost identical outfit, right down to the skinny navy jeans and wedge shoes, when she attended a volleyball session at London's Olympic Park in October.
The top's page on the Me+Em website had a message today reading: "Sold out due to popular demand - more stock arriving soon."
It described the top as "a perennially chic style", featuring "flattering" features like a scoop neckline, three-quarter length sleeves and "ruched detailing to the bottom hem".