Workers claim piles of undelivered parcels are stacked so high at a new Evri depot that they have fallen on workers.
The Health and Safety Executive (HSE) has been called in to investigate the claims, and bosses have been told to make changes.
One staff member, who wanted to remain anonymous, claimed packages had been 'falling' on workers and that employees had been forced to sort parcels outside instead of inside the depot as a result.
Evri has denied this.
The Evri staff member told PlymouthLive that staff shortages resulted in some packages were being piled up inside the warehouse.
Photos handed to the website showed parcels both inside and outside the depot in Plymouth, which opened in July.
An HSE spokeswoman said: “HSE routinely visits businesses as part of its regulatory function and can confirm we recently visited the Evri depot in Plymouth.
"We are not aware of [falling parcels] happening, there have been no reported accidents and this did not form part of the discussions with HSE."
An anonymous worker said the situation inside the depot had "improved" since the HSE visit.
They said: "Somebody complained to HSE about the stacking and cages inside the depot. Ever since it's like they don't want us inside. But the way parcels are now being stacked is much neater.
"There's a different section where customers' parcels are everywhere.
"It's not a backlog, but parcels where you have something like an incorrect address. This can be rectified in minutes but there are not enough staff to do so, so it's all piled up inside.
"Packages would fall on you and heavy boxes are stacked on top of the cages, not at the bottom."
An Evri spokeswoman said: "We can confirm that we have implemented a number of changes following a meeting with the HSE as the health and safety of our people remains a top priority."
They previously said that due to capacity issues as a result of Christmas volumes, couriers were sorting their parcels outside in an adjacent car park where there is safely room for 27 vehicles off the road at one time.
