"It's hard to put any other way. I start to get frustrated if I don't have some creative outlets," he said.
"I can go for quite a period of time without composing but I find I'm composing most of the time, even when I'm busy with teaching.
"I think it's a real need to get something out."
One of the most prolific New Zealand composers, Dr Ritchie says he finds it more exciting to create something new rather than reinterpret, as performing artists do.
"Plus I want to express ideas I can share with people. That's important to me - to feel I can say something that other people may find interesting or thought-provoking."
He says in recent years he has developed a political awareness, writing pieces such as Whale Song to protest against whaling.
"It probably wouldn't make a blind bit of difference but it's something I feel I want to do."
Ritchie's Symphony No 3 will be given its world premiere by the Southern Sinfonia on October 9, the opening weekend of the Otago Festival of the Arts, a few weeks after his 50th birthday.
Writing an orchestral symphony is the ultimate challenge for a composer, he says.
It's a complex task to produce some 30 lines of music for more than 50 instruments, but he prefers to think of it like using different colours on an artist's palette.
"It takes a long time to be able to think in terms of all those colours, all those instruments."
Ritchie develops his ideas at the piano, then translates them into the sounds of the orchestral instruments.
With experience, he is able to keep those sounds in his head while working on others.
He also likes to keep particular players in mind and write solos for them if he knows the orchestra that will perform it, he says.
The Southern Sinfonia has performed his previous two symphonies. No 2 The widening gyre, inspired by W. B. Yeats' poem "The Second Coming", was written to celebrate the millennium.
His first, Boum, premiered in 1994, was inspired by E. M. Forster's novel A Passage to India.
The new symphony has been a couple of years in development. It has a psychological base, dealing with mood swings and trying to find some sort of balance via relationships with other people and keeping in touch with reality, he said.
Its two movements are called simply Up and Down.
"The first movement is almost hyperactive. It's very full-on, fast and furious, and it's actually quite a challenge to sustain that for 14 or 15 minutes.
"Obviously the second movement is slower and more brooding. It goes through different states. The opening is rather static and numbed. Then it bursts into more angry, frustrated passages.
"It goes through different states like that, and eventually, the coda, the end part of the work, finds resolution, and it is musical resolution as well as psychological, so there's some sort of balance about the piece," Dr Ritchie says.
Believing orchestras need to evolve, he likes to introduce instruments not normally found in them - in the first symphony he used a saxophone, and in this he introduces a log drum of the type used by Cook Islands drummers, and a keyboard, either electronic or an organ.
Dr Ritchie, who grew up in Christchurch, has lived in Dunedin since he was Mozart Fellow at the University of Otago in 1988 and 1989. He had not intended to stay on, but he and his first wife separated towards the end of the period and he decided to remain to share custody of their son Tristan.
"That was the initial reason, though I hadn't really planned to stay.
"In the long run though, it proved to be good for me because I established myself and slowly created a freelance career for myself here and made a lot of new friends.
"I actually like living in Dunedin. It's a good place to be a creative artist. Dunedin is small enough so you get to know people but it's not too small and there's plenty happening here.
"I make a point of travelling quite frequently during the year, going up to concerts and events and things, just so I don't feel I'm missing out on what's happening up there."
For the past 10 years he has been teaching composition at the University of Otago, which gives him time to compose music.
Although he still does some freelance work, he no longer has to work to demand and can compose what he likes, he says.
Dr Ritchie comes from a musical family. His father John Ritchie, emeritus professor of music at Canterbury University, is also a composer.
"In terms of the type of music he wrote, that didn't influence me directly - I had other ideas I guess, but he was a role model and the concept of being a composer didn't seem strange, seemed quite a natural thing for me to do.
"But I think his influence came about through all sorts of practical advice he gave me about scoring, writing for instruments, the best way of making the timpani sound good - that type of thing, and just being interested and someone to talk to about composing."
Dr Ritchie's own children love music, and though his grown-up son Tristan is not interested in composition, his 11-year-old daughter Annabelle plays instruments, and he has noticed she has written the odd little tune - "so you never know", he says with a laugh.
• FIRST TIME
Anthony Ritchie's Symphony No 3 will be premiered in the Southern Sinfonia's concert in the Dunedin Town Hall at 8pm on October 9. The programme includes John Ritchie's Papanui Road overture, which hints at the bustle, the vitality and the peace of Papanui Rd in Christchurch, and Beethoven's Violin Concerto with Bulgarian violinist Bella Hristova, winner of the 2007 Michael Hill Violin Competition. Conductor is Simon Over.
