In 1999, Nasution made his official debut, as a 16-year-old, at the Southeast Asian Games in Bandar Seri Begawan, Brunei, along with his sister Elsa Manora Nasution. Although he did not consider a heavy favourite, Nasution earned a bronze medal in the 200 m breaststroke at 2:20.59, just a full body length behind Malaysia's Elvin Chia and Thailand's Ratapong Sirisanont.