High Court's tied decision offered little legal precedent on the extent that offensive speech can be prohibited in Australia.
She said the issues could be tried again in a different case. Two of the seven judges on the High Court will have changed before the next such case is heard.
"It's rather unpredictable'' how the court would rule on a similar case, Twomey said. "The area of offensive speech has always been difficult.''
Only six judges heard the case because the seventh, Justice William Gummow, intended to retire in October last year before the trial was likely to be completely heard.
One of the judges who would have upheld the appeal on free speech grounds, Justice John Dyson Haydon, retires in March.
It is a crime under Australian federal law to use a postal service to communicate a message that "reasonable persons would regard as being, in all the circumstances, menacing, harassing or offensive.''
Twomey said Monis and Droudis might not have been charged if the letters had been hand delivered.
Federal authorities have limited criminal jurisdiction in Australia and relied in this prosecution on its powers over national postal and electronic communications.
Australia has 1,550 troops in Afghanistan which is the biggest military contribution to the war of any country outside NATO. Australia has suffered 39 casualties over the past decade in Afghanistan and another 249 Australian soldiers have been wounded.