Whistleblower David McBride has made a shock decision on the eve of his trial

Whistleblower David McBride, who provided classified documents to journalists on secret allegations of Australian war crimes in Afghanistan, has changed his plea.

Former military lawyer David McBride who helped exposed alleged war crimes by Australian soldiers in Afghanistan has pleaded guilty just days before his criminal trial was to get under way.

He has pleaded guilty to three charges of three charges of stealing and unlawfully sharing secret military information.

He changed his plea on Friday after the ACT Supreme Court ruled he had no legal duty to defy orders that were against the public interest.

David McBride (left) with his legal team arriving at court in the ACT this week. Picture: NCA NewsWire / David Beach

Mr McBride had planned to rely on the oath of service he swore to the Queen when he joined the military for his defence.

But Justice David Mossop found Mr McBride had no legal right or obligation to breach orders, and his actions were not justified by public interest.

Mr McBride had admitted to providing the classified documents to the ABC, which used them to produce of series of report in 2017 called the Afghan Files.

The landmark Brereton inquiry later found evidence that Australian forces had unlawfully killed 39 Afghans during the war.

His trial before the ACT Supreme Court had been due to start on Monday.

Mr McBride will now be sentenced in early 2024.