Unkles told the ABC he believed Roxon had made a political, rather than legal, decision.
"They don't want to offend the British and there are powerful institutions and individuals in this country who want to make sure that this case goes no further ...
But Unkles' claim that Morant, Handcock and Witton were not guilty because they were ordered to kill the prisoners has been disputed by other experts.
Fellow historian Craig Wilcox told the ABC that the three soldiers had not denied the shootings, no evidence of any orders to kill the prisoners had been found, and one of Morant's own men said any such claim was a "bogus idea". He also said the trial lasted about five weeks in an age when civilian murder trials typically took only three days, and had been thorough and fair by the standards of the day.
In a letter to Unkles, Roxon said it would not be appropriate for the Australian Government to advocate a pardon when there was no dispute that Morant and his colleagues had committed the killings of unarmed Boer prisoners and others.
"I consider that seeking a pardon ... could be rightly perceived as 'glossing over' very grave criminal acts."