The lawyer for a Canadian diplomat called to testify at a hearing into allegations of Afghan prison torture says the Conservative government is trying to keep her client silent.

In a letter sent to the Department of Justice and obtained by CBC News, Lori Bokenfohr said the government has invoked the national security order in response to Richard Colvin's decision to co-operate with the Military Police Complaints Commission.

The commission is examining whether military police officers had a duty to investigate the transfer of detainees when there were allegations of torture in Afghan prisons by Afghans.

But the order prohibits Colvin from telling the commission what he knows.

In the letter, Bokenfohr questions the government’s use of the orders, saying they were not "intended to be used tactically to intimidate witnesses from giving evidence in administrative proceedings carried out by government-created bodies."

"We raise these issues to emphasize that the interests of justice are not served when an ordinary witness such as Mr. Colvin is threatened by the Department of Justice with severe penalties for abiding by the terms of a subpoena served on him," Bokenfohr writes.

CBC News earlier reported that Colvin was the author of a 2007 report that contained claims of torture from Afghan detainees.

The report said one detainee claimed to have been beaten with cables and wires and electrocuted.

Another detainee said he was beaten and forced to stand for two days during interrogation. All the abuse was allegedly carried out by Afghans in Afghan prisons.

The report was widely circulated at the senior levels of the defence and foreign affairs departments.

But Defence Minister Peter MacKay has said the government has done nothing wrong in its dealings with the commission process.

"The commission is proceeding with its important work. We have provided thousands of documents, we have co-operated with witnesses, within the mandate of the commission," MacKay told the House of Commons.

NDP defence critic Jack Harris said the Conservative government is trying to keep Colvin from speaking about the abuse of Afghan detainees for fear of what he might say.

"The indication seems to be that much of this evidence might embarrass the government because it might reveal what they knew, when they knew it and whether they did anything about it or not."