The occasion was a function hosted by Prince Philip for 30 or 35 guests, patrons of The Outward Bound Trust, and was at Frogmore House in Windsor on March 5, but they were not allowed to tell the media before the event.
"It's such a unique thing. I don't know of any other New Zealander who's been in such a situation, apart from Dame Kiri herself.
"There we were representing the country in front of Prince Philip and all his guests.
"He looked amazingly dapper and still very dashing.
"His attendant said that at 90, after that little heart thing he had, he's looking better than ever."
It was a rare honour to be asked to do a command performance, because the Royal Family did not have private recitals as often as they used to, Prof Dennis said.
Frogmore House has been used as a royal residence since Queen Charlotte, wife of George III, bought and extended it in the 1780s, although now it is used mainly for receptions.
"Our retiring room before the concert was the Duchess of Kent's sitting room, which is closed to the public.
"The Duchess of Kent was Queen Victoria's mother and this room was filled with Queen Victoria's own artefacts, including a piano and beautiful portraits of the children and herself and Prince Albert - and her piano duet book was there on the table."
The great drawing room in which they gave the recital had been a ballroom.
The furnishings were in white and gilt with portraits of royal figures on the walls, and behind the piano were Wedgwood candelabra, he said.
"It was like walking into another century. It looked like something out of George III's time. Handel could have been in the room!"
The recital consisted of works by Handel and Vivaldi, songs by Liszt in French and Italian, Canteloube's Chants d' Auvergne for which Dame Kiri is famous, and a selection of English folk songs and a song by Argentine composer Ginastera, Prof Dennis said.
Dame Kiri and Prof Dennis have given many recitals together over the past few years.
In Dunedin they gave one two years ago in the last Otago Festival of the Arts.
Last year they gave recitals in Hawaii, Melbourne and Brisbane and this year will be doing another five in Australia.
Prof Dennis is a trustee of the Dame Kiri Te Kanawa International Foundation, and has also taught at the Solti-Te Kanawa Accademia di Belcanto in Italy.
Dame Kiri now spends a lot of time mentoring young singers, but still gives recitals, although no longer sings in opera, Prof Dennis said.
And, no, artists did not get paid for giving Royal Command performances.
The university paid his fare to London because it was such a prestigious event, he said.