It has asked the government in Colombo to help find the man who it says may have been abducted.
The political activist and his female colleague vanished in separate places three days before they were to launch a leftist political party.
Their party colleagues say the government is to blame.
Premkumar Gunaratnam has had a chequered history, living for years in Australia, taking its citizenship and adopting an alias after taking part in a failed leftist insurrection in Sri Lanka in the late 1980s.
But it was in the Colombo suburbs that he and his female colleague, Dimuthu Attygalle, disappeared on Friday night, days before they were to spearhead the launch of a new left-wing party.
Premkumar’s mother, VR Gunaratnam, told a news conference that he had been abducted by unknown people and his life was in danger.
“I want to see him. I want to see him back. And I ask the public to help as a mother. A mother’s voice, I think it will go all around the world – to help my son, to release my son,” an emotional mother appealed.
Their party comrades say the two missing people were abducted by forces linked with the Sri Lankan government because the party campaigns on human rights issues.
Australia says it is concerned. A spokesman for its Department for Foreign Affairs and Trade told the BBC it was seeking to clarify the missing man’s situation with the Sri Lankan authorities.
It did not name him but said its top diplomat in Sri Lanka had asked for government help in finding him.
The Sri Lankan military spokesman described allegations against the government as “wild”.
“We do not even know whether the people have disappeared,” he commented.
Human rights workers here say dozens of Sri Lankans have now disappeared since October.
Sri Lanka’s war ended three years ago but government figures and media outlets often describe dissenters as traitors.