Travel agency Flight Centre has mistakenly released the personal information of some of its customers to "third-party suppliers", in an incident it describes as human error rather than a security breach.
The ABC understands the leaked information included passport details.
Flight Centre would not say how many customers had been affected and would not clarify what information was released.
A company spokesman said they had acted quickly to contain the information when it became aware of the issue.
"We also sought and received assurances from the suppliers that they did not retain copies," the spokesman said in a statement.
He said the company had contacted affected customers.
"While we believe the risk that this information will be misused is relatively low given the circumstances, we chose to inform the affected customers so they could take additional precautions if they considered it appropriate," the spokesman said.
"The steps people should take have been outlined in the letters they have received."
Customer Nikki Racco has told the ABC she received a letter from Flight Centre.
"[The letter told] me they had mistakenly made some customers' details available to a third party, which included revealing our passport numbers," she said.
"[I] am absolutely dismayed that once again we are 'forced' to provide this information as part of a service and yet there can no longer be any expectation that this information can be kept safe."
