They used to call it the Kate Middleton effect. Any dress or outfit worn by the Duchess of Cambridge for a photo shoot or to an event would often sell out within hours, sometimes even minutes.

Now we have the Melania effect.

Melania Trump, the wife of presumptive Republican presidential candidate Donald Trump, wore a dress by Serbian-born designer Roksanda Ilincic to address the Republican Convention in support of her husband in Cleveland, Ohio, on Monday US time.

The dress, which cost $US2190 ($2914) on online retailer Net-a-porter.com, sold out in some sizes within hours of Mrs Trump's appearance. On Tuesday night, it was showing sold out or "low stock" in all sizes.

The Roksanda 'Margot' dress, as worn by Melania Trump. Photo: Net-a-Porter.com

It's not the first time Ilincic has benefited from such a high-profile appearance. The label is also a favourite of the Duchess of Cambridge, who wore a couple of Roksanda dresses on her tour of Australia with Prince William in 2014, and Michelle Obama.

Time magazine speculated that Mrs Trump chose the dress for Ilincic's Serbian origins. Mrs Trump was born in Slovenia, which together with Serbia was part of the former Yugoslavia.

According to fashion bible Women's Wear Daily, Mrs Trump chose the dress herself and did not work with a stylist to choose her look for the convention.