Recently she has been working with students at London's National Opera Studio and in Cardiff at the Welsh International Academy of Voice.
Professor Cesar Ulloa is chairman of voice at the San Francisco Conservatoire and also leads programmes for the San Francisco Opera Center. His students have been first-prize winners and finalists in numerous competitions including New York's Metropolitan Opera, Placido Domingo's Operalia, Concurso Montserrat Caballe and the Cardiff Singer of the World.
A spokesman for the Juilliard School of Music in New York said Ulloa offered incredible "insight into the foundations of great vocal artistry". He has been described as "a guardian angel of singing" and "an invaluable and important part of the opera world".
He is considered by many to be the best singing teacher in the world, says NZ Opera School director Jonathan Alver.
"He came to teaching from a long and successful opera stage career and says that hearing stars such as Caballe, Carreras and Kraus, both on stage and in rehearsal, trained his ear to hear where young singers will probably be two years ahead."
He and Della Jones were spectacular vocal coaches for this year's 24 students, he said.
The Opera School - held in Whanganui every January - received a ringing endorsement from New Zealand's legendary Dame Kiri Te Kanawa last month.
Last January, international opera star Dame Kiri visited the school for the first time to see first-hand the institution that her own foundation had supported financially for the past five years.
"I very much wanted to see for myself just what it was like," Dame Kiri said in a letter to the Opera School. "I was not disappointed - the facilities are superb and the final concert was of the very best quality."