Hoarding in response to the economic downturn and the ease of selling recycled designer clothes online are being blamed for a dramatic slump in donations to op shops across the nation.
St Vincent's is down more than 100,000kg, or 20 per cent, on clothing donations in WA this year compared with 2008-09.
Spokeswoman Lucinda Ardagh said the shortfall was a big blow to the charity.
Vinnies relies on its retail shops to provide 40 per cent of its income.
Ms Ardagh said the items that were coming in were older and more used than in previous years, with many items so stained and damaged they could not be on-sold and had to be dumped in landfill at an added cost to the charity.
Sean Burgess, of the Salvation Army, said donations were down 10 per cent on the same period last year, while the theft of donated goods left outside op shops after hours further reduced available stock.
National Association of Charitable Recycling Organisations chief Kerryn Caulfield said stocks held by Australia's 2000 charity op shops were at record lows.
"When the economy is tight, most people hold on to their possessions, yet demand for charitable assistance increases," she said.
Online action group Do Something has launched a campaign to encourage people to clean out their cupboards and take quality items to their local op shops during National Op Shop Week, which starts on Monday.
Founder of Do Something Jon Dee said Australians had billions of dollars worth of unused clothes that could help the needy.
He said more than 90 million kilos of donated goods were resold through op shops each year but almost as much was sent to landfill because it was unusable or broken.
"The campaign is calling on people to have a spring clean and give their good quality unwanted clothes to their charity op shop," Mr Dee said.