Customers expressed outrage on the company's Facebook page on Thursday when they learned they wouldn't be reimbursed their funds for three business days - a total of five days out of pocket when the weekend is factored in.

One distressed customer said she had run out of medication which she needed as a matter of survival, and received "no assistance" when she contacted the company.

Another customer published a snapshot of his bank account showing he had been (temporarily) robbed more than $1000 as a result of the duplicate transaction.

Others users complained they had been on hold to PayPal for up to two hours on the phone with no response.

Comments were flooding onto the Facebook page from early on Thursday, several hours before PayPal finally published an official statement on the page.

It told customers they didn't need to contact the company as they would automatically be reimbursed in three business days.

PayPal Australia would not respond to Fairfax Media's questions as to how many were affected other than to say it was aware of the error and would reimburse all affected customers.

"We apologise to those that have been impacted and are actively working with our tech team to resolve this as quickly as possible," the company said in its statement.