"I felt I had discovered a role I just loved singing. I'm enjoying moving into old-man bass roles; kings and all that stuff. Wagner wrote Marke's music so beautifully. It's very lyrical - a pouring out of gorgeous tone and emotion that inspires you to sing your best."
Whelan's association with Bach's St John Passion goes back to his childhood in Christchurch where his choral director father Don Whelan "has put this work on religiously every Good Friday for the whole of my life. I remember, at 5 or 6, falling asleep during it, but eventually I sang in the choir and then took solos".
It was this work that gained Whelan international attention when he appeared as Jesus in Deborah Warner's 2000 staging of the piece for English National Opera.
"I had long hair and a beard back then, which is probably why I was cast." Critics praised him as "visibly and audibly right" with "a performance of real stature".
"Our entire culture has grown out of the Passion story. It's a very poignant narrative. When I had to play Christ in a dramatic production I had to find a way to convince myself why I was singing these works. That meant I did a lot of reading and thinking."
More recently, in Australia, Whelan has been taking part in Lindy Hume's dramatisation of Bach's St Matthew Passion, reaping critical praise for his "charismatic and richly-toned Jesus".
The St John Passion is the more compact of the two works and the APO presentation will run at just under two hours - conductor Stephen Layton, who worked with Whelan on that 2000 ENO production and was responsible for the APO's splendid Bach B minor Mass last year, has insisted that there be no interval.
"Jesus is constantly asking questions in the St John Passion," says Whelan when we talk about characterisation, "questions that are directly related to life and existence, the same sort that might still be asked today.
"But then many feel that John's Gospel is the most authoritative of the four. The most likely to have been connected with someone who was actually there."
As for Bach's music, he finds he reacts to it on a very physical level. "It's incredible," he exclaims. "The effect that Bach creates with the rhythmic surge that runs through the work is almost hypnotic."
What: Auckland Philharmonia Orchestra - St John Passion
Where and when: Auckland Town Hall, Thursday at 8pm