Kumar Gunaratnam is a member of Frontline Socialist Party (FSP). He has been sentenced to one year in jail and slapped with a fine of Rs 50,000 by the Kegalle Magistrate's Court.
Gunaratnam, 49, was arrested by the police in November last year for overstaying his entry visa.
The FSP leader returned to the island in January 2015 in time for the presidential election and had remained in the country illegally. He extended his 30-day tourist visa since January 31.
The FSP is a radical socialist party is being accused of controlling the students unions in universities and carrying out recurrent political agitations on the streets.
His party's plea to accord him dual citizenship status was rejected, party source said.
It was not immediately clear if he would be deported to Australia after he had served the jail sentence.