The New Zealand singer who co-founded opera band Amici Forever is giving up the group to devote more time to his newly diagnosed autistic daughter.

Geoff Sewell, a former accountant-turned tenor, says he has made the "difficult" decision to put his family first. The "popera" group's tour of New Zealand and Australia next month will be his last.

Sewell, 31, says 2-year-old Sienna's condition means she needs routine and stability, which are incompatible with the band's globe-trotting tours.

The diagnosis came last month when London-based Sewell and his New Zealand wife, Simone Lanham, also parents to 4-month-old Olivia, saw experts to find out why Sienna was not speaking.

After weeks of tests, doctors "dropped the bombshell", says Sewell. "And that completely ripped us apart. We didn't know what it was all about.

"Simone was the first to speak after we heard the news, and she asked: 'What does her adult life look like? Will she be able to have a relationship, get married, have children like other people?' The doctor said: 'To be brutally honest, it's highly unlikely'."

Five weeks after the diagnosis, "we're still bursting into tears - even when I was running to work this morning. Why us, why this?"

Sewell says the decision to leave the band was "difficult and painful" but did not take long.

Doctors have stressed that the input Sienna receives in her first five years is crucial, and Sewell is using pictures to help his daughter develop communication skills.

He is meeting Autism NZ representatives while touring here, to raise the profile of the disorder.

It was a family crisis that sparked Sewell's move into a musical career. His sister, Julie, died aged 21 during an epileptic fit.

He recalls from her experience the ostracism that people with "social disorders" can face.

"I'll be doing my best to be in the playground protecting Sienna and making sure people aren't nasty to her, because that's what kills me."

After Julie's death, Sewell reassessed his career as an accountant and decided to follow his heart, training at the Boston Conservatory in the United States.

He created a troupe of singing waiters, Tenors Incognito, in London in 2000, performing at celebrity parties.

In 2002, Sewell started Amici Forever, made up of himself and four other classically trained singers who fused classical music with pop pizzazz.

Their debut album, The Opera Band, sold more than a million copies. The second album, Defined, topped the Kiwi charts.

"It's all been an unbelievable pace."

The Sewells no longer have a stake in the band but Tenors and Divas Incognito, which has offices in London, New York and Edinburgh, will continue to provide income.

The couple hope to open offices in Australia and New Zealand soon as part of a plan to eventually head homewards. Sewell grew up in Hawkes Bay.

Amici, now a foursome, will be joined on their New Zealand tour by singers Russell Watson, Yulia and violinist Ben Morrison. The band will continue with a replacement tenor after the final Australian show.

But this is not the end of Sewell's career. "When I want to come on tour I'll pick the time that's right for the family."

What is autism?

* A life-long developmental disability affecting social and communication skills. People have difficulty making sense of the world.
* Affects 40,000 New Zealanders, or 1 per cent.
* The causes are still unknown but research shows that genetic factors are important.