Financial misconduct claims have hit Australia's outsourced diplomatic post service.

International courier company DHL has reportedly referred a matter involving three of its employees to the Australian Federal Police.

Private contractors working on Australian Government departmental freight services have been stood down, but the Department of Foreign Affairs and Trade (DFAT) says its contractual arrangement with DHL is continuing amid the investigation.

“DHL has advised it has taken action after an internal DHL investigation identified alleged financial misconduct by contracted staff providing mail and unclassified diplomatic freight services to the Department of Foreign Affairs and Trade,” a DFAT spokesperson said.

“DFAT will continue to work with DHL to ensure appropriate contract oversight and strong internal controls are in place.”

It has been alleged that the issue involves suspicious billing arrangements.

The Federal Opposition said that if the allegation of misconduct were proven correct, it would be “incredibly concerning”.

“It's essential to allowing our embassies and diplomatic officials to function effectively and discretely around the world and if that's been compromised by private contractors, then that's obviously a significant issue,” Labor spokesperson Pat Conroy said.