He describes his time in Whanganui at the opera school as "very happy and fulfilling years".
"I love coming back here to this town where the people made me feel like a rock star,'' he laughs.
It still staggers him nearly four years later that within days of leaving the 2017 school he was head-hunted by acclaimed Belgian composer Fabrizio Cassol.
Cassol had spent months searching the world for a fine counter tenor for his collaboration with Les Ballets de La B in Brussels encompassing a spectacular blend of music and dance choreography, when he discovered the 27-year-old Diaz.
"He found me through a video on the opera school website: it had only been up a few hours. That was it, I was off to Belgium within a week … it was mad, wonderful … I felt amazing.''
But coming home earlier this year in March to see his family in Auckland didn't quite go to plan.
"Well, of course I was stuck in the Covid-19 lockdown. It actually became a huge blessing. For the first time I could concentrate completely on my voice … really work hard on my technique, for me it became personal work of the highest calibre and I am so grateful."
Continuing in Covid vein Stephen laughs when he said that last month when Auckland went into a second lockdown during the opera rehearsals for Semele they had to rehearse wearing masks.
"We had no choice and there we all were singing through our masks … it was very weird."
However, wearing masks meant the singers had to use their eyes only to illustrate their facial expressions.
As in every opera fine acting as well as singing is vital, he says.
"Our acting improved out of sight after the two weeks of wearing masks … unbelievable. Though we laughed about it we knew we had made incredible progress without realising it."
Stephen will return to Europe the day after Semele closes at the start of November to pursue auditions across Europe and Britain.
Though he is eager to return he said the competitive, fierce auditions are very difficult.
"There are so many fine singers … hundreds of them something you don't see in New Zealand. It's always so fabulous to come home for a while."