In the past financial year, Red Cross spent approximately $50,000 in rubbish tip costs to dispose of donations unable to be sold.
"What we've observed is an increase in the amount we can't sell," O'Leary said. "We're trying to slow down the fast fashion cycle by giving items a new wearer and re-presenting it to the market again, but it's a challenge."
Donated clothing from certain retailers would never make it on to Red Cross' shelves, she said, although she would not name which retailers.
"But then, at the high-end of the market, we've got some partnerships with mainstream retailers in which we take any returns or garments that they've overstocked and have not been able to sell, and therefore we're preventing that from going directly to landfill," she said.
She would not disclose which retailers she was talking about.
Retail consultant Chris Wilkinson said the short lifespan and increasing disposability of clothing should be a concern for everyone.
"While we're actively working to stem the impact of packaging and plastics on our environment, little thought is given to the huge amounts of waste textile joining our landfills - along with the associated impacts of production, freight and other aspects of bringing these products to market," Wilkinson said.
"Low original prices of these garments mean consumers rarely expect to pay much, if anything, for those in charity shops and increasingly donations are having to be sidelined."
We're trying to slow down the fast fashion cycle by giving items a new wearer and re-presenting it to the market again, but it's a challenge.
Wilkinson said he believed fast fashion was here to stay.
"Consumers [are] increasingly blending cheaper 'staple' clothes with more expensive 'signature' pieces - such as a cheap T-shirt with a pair of G-Star jeans.
"Retailers are telling us fashion labels get less of a look-in than before, where consumers would have bought more of their clothing from brands, they're now happy to mix cheap with chic."
Recycle Boutique New Zealand operations manager Mark Cowie said he had noticed an increase in mass-produced clothing donations.
"We try to be specific about what we put into our stores but we are seeing a lot of mass produced items, it's good in a way as its protecting our landfills a little bit longer by recycling," Cowie said. "But we [now] have to go through a lot more product in order to get sellable items, which takes a lot more time."
The retailer, which has eight locations around New Zealand, was slowly phasing out accepting mass-produced items, Cowie said.
"We see a lot more fast fashion than we did three years ago."