"The stuff was blocking our fire exit - we couldn't even open the back door. We are short-staffed and it will take about a day's work to sort through everything."
With four drop-offs and an influx of customers within 15 minutes of the shop opening on Monday morning, Ms Sibbald said she and her volunteers were under pressure and the dumped goods meant she needed to call in extra staff.
"People think they're helping by leaving stuff but they're not helping. It's causing us a massive problem by leaving it there with no one to accept it," she said.
"It costs us money to get rid of the goods that we can't sell in the shop. It is $80 to empty our rubbish bin and we have filled that up with the dumped goods.
"People need to realise that if they drop off goods when we're not here, we never receive them as they intended because other people go through them and dump things on the ground. It means we often have to bin perfectly good items because they're no longer usable.
"That said, we are hugely grateful to the community for their support and the ongoing generous donations we receive."
Ms Sibbald said the shop is back to its usual opening hours but will be closed on Saturday, January 20, as staff will be selling retro items and collectables at Vintage Weekend.
The Hospice Whanganui shop in Gonville has ongoing problems with people dropping off items out of hours, manager Sue Lumsden said.
"That happens to us all the time and a lot of it is good stuff when it's dropped off but then people go through it and spread it everywhere," she said.
"We won't sell it if it's been gone through and we then have to pay for dumping it. I think people don't understand that we can't sell stuff if it's been damaged."