The Maureen Forrester Prize is awarded to the second-placed entrant in the competition.
First prize went to baritone John Brancy, of the United States, and third went to mezzo-soprano Clara Osowski, also of the United States.
Van Mellaerts (29), who is now based in London, was one of 38 young singers from around the world selected to participate in the event this year.
The former University of Otago music and languages student recently graduated from the Royal College of Music International Opera School after receiving financial support from the Kiri Te Kanawa Foundation.
"I am thrilled to have come away from CMIM 2018 with the Maureen Forrester Prize as well as the German Lied Award for best performance of German song.
"It was a hugely rewarding experience and I learned so much."
Dame Kiri Te Kanawa, who was a judge at the competition, said she was very proud to see a New Zealander compete so successfully against an international group of outstanding young singers.
"He was very much admired by my fellow judges.
"He had a programme of a well-thought-out group of songs and his performance was really beautiful.
"The most annoying thing for me was that I could not vote for Julian as the foundation supports him, so there was a conflict of interest."
Van Mellaerts' success caps a year of outstanding achievement.
He won first prize in the 2017 Kathleen Ferrier Award and first prize at the 2017 Wigmore Hall-Kohn Foundation International Song Competition and was awarded the Tagore Gold Medal at his graduation from the Royal College of Music.
He will return to New Zealand later this year to take the role of Schaunard in New Zealand Opera's performance of La Boheme, in Auckland and Wellington.
