Australia's diplomatic mail has been caught up in allegations of financial corruption, after private contractors working on departmental freight services were stood down for alleged misconduct.

Key points: The ABC understands DHL's CEO flew to Canberra in January to give DFAT assurances about the integrity of its diplomatic mail service

The ABC understands DHL's CEO flew to Canberra in January to give DFAT assurances about the integrity of its diplomatic mail service The Federal Opposition says it would be "incredibly concerning" if the allegations of misconduct were proven correct

The Federal Opposition says it would be "incredibly concerning" if the allegations of misconduct were proven correct DHL confirmed the mail and diplomatic freight services they provide to DFAT have not been impacted

The ABC has learned that international courier company DHL has referred a matter involving three of its employees to the Australian Federal Police.

In a statement, the Department of Foreign Affairs and Trade (DFAT) said its contractual arrangement with DHL was continuing as an official investigation is carried out.

"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."

"DFAT will continue to work with DHL to ensure appropriate contract oversight and strong internal controls are in place," a DFAT spokesperson added.

The ABC understands no diplomatic parcels were interfered with but billing arrangements were found to be suspicious.

A source familiar with the internal DHL investigation told the ABC the company's regional CEO, Gary Edstein, flew to Canberra in January to give DFAT assurances about the integrity of its diplomatic mail service.

"DFAT will not incur any financial loss as a consequence of the alleged misconduct," the Department told the ABC.

The Federal Opposition says it would be "incredibly concerning" if the allegations of misconduct involving diplomatic mail services are proven correct.

"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 spokesman Pat Conroy said.

In a statement, DHL insisted it was "fully assisting" authorities in their ongoing investigations of the matter.

"As an official investigation is currently underway, DHL is unable to discuss or disclose further details but can confirm the mail and diplomatic freight services by DHL to DFAT have not been impacted," a company spokesperson said.

Labor said it would be seeking further information from the Government over what steps had been taken following the discovery of apparent misconduct connected to Australia's diplomatic mail service.

"We have seen time and time again contractors within DFAT compromising essential operations and DFAT has to fix this," Mr Conroy said.