Hong Kong-based NGO Redress says it has collected close to 18 tonnes of unwanted clothing in the last month.

The mountain of clothes, from over 120 collection points across the city, was put on display at the group's "Sort-a-thon" event on Thursday.

Volunteers will spend three days sorting through the items and deciding what should be resold, donated, downcycled or thrown away.

Executive director of Redress, Nissa Cornish, said it is estimated that 402 tonnes of textiles are discarded every day in Hong Kong.

"It really opens your eyes as to how big the actual problem is in Hong Kong, this is just a tiny drop in the ocean," she said, referring to the 18 tonnes of clothes collected by the group.

"We are also seeing an uptake in people participating in campaigns like this one, shopping second-hand or swapping [clothes], especially among younger people...there is a trend towards more sustainable clothing habits in Hong Kong," Cornish said.

"When people ask us what is the most sustainable thing they can buy or wear, it is what's already in your closet. Even if what you already have in your closet is old fast fashion."