Hayley Westenra is getting over her jetlag when we catch up, having just returned from an intensive month-long tour of Japan before jetting off to the States to sing to 300,000 people in Washington, and returning to Henley in time for her performance tomorrow.

"Is that how many people it is?" she asks completely unperturbed.

"I thrive on the crowds and the energy they give off. But then I've always been as comfortable as a performer and a singer, they go hand-in-hand to me. Maybe it's because I started performing at such a young age."

Having recorded her first album aged 12, Hayley has music in her blood, her grandmother also being a singer and her grandfather a pianist. But no one was prepared for Hayley's instant worldwide success once her album Pure hit the shops in 2003.

The soprano has now achieved sales of well over three million albums around the world, and sung with Jose Carreras, Bryn Terfel and Andrea Bocelli to guests including The Queen and President Bush at venues such as Wembley Arena and the Royal Albert Hall.

But right now all she wants to do is "chill out at home and eat Vegimite on toast on the sofa because I feel like a zombie.

"I haven't even got the energy to text my friends. But other times I'll get home and phone all my friends and go out."

Which takes us neatly on to shopping and all the frocks she needs to buy: "My favourite dress at the moment came from a shop in Houston, Texas, where Beyonce goes and it's just stunning.

"But the problem is that while you want to wear the same thing over and over again, you can't. So yes I do enjoy the dressing up as long as it reflects my personality."

So does she get time to shop? "Sometimes I have a stylist to go and choose me some things and hunt them down or otherwise I'll be in a random city and just come across a store that I love," she says.

And yet this world of red carpets, royalty and designer gear is a million miles away from her charity work in Africa.

As the youngest UNICEF Ambassador in the world, Hayley has worked in Ghana and is returning there in September to raise awareness for playpumps to bring clean water to villages.

And she finds moving between these two worlds "quite difficult, because one week you're staying in a posh hotel and the next you're in Ghana, walking around the slums, which is pretty traumatic and draining.

"Coming back afterwards is hard and also scarily easy because you want to tell everyone about the project but it's amazing how quickly you slip back into your old life," she explains.

"So you just have to do what you can and I'm not going to help much by giving up my singing career. Singing is how I can help and raise awareness."

In terms of maturity Hayley is 20 going on 60. But this New Zealand prodigy always had her head firmly screwed on.

"Sure I have had my moments of wanting to go out and forget my responsibilities. But aged 16 it was harder because I was much more keen to be social and not miss out on any parties.

"Now that I'm 20 I've realised I can have fun while I'm working. So I have a great time while I'm on tour and it doesn't really feel like work. I just have to mix the two.

"And while my parents used to accompany me everywhere, now they don't come to keep an eye on me, but to catch up. It was an easy transition because my parents just realised they weren't needed as much."

Hayley also puts much of her success down to her management team allowing her to "develop as a performer and be the creative driving force rather than clipping my wings. They believe in me and don't want to just replace me with a new voice."

Ask her legions of fans and they'd agree that replacing Hayley would be nigh on impossible. But before her star gets any brighter, you can spot her at the Henley Festival tomorrow night. "I'm really excited about that," Hayley adds. "Because I've never appeared there before so let's hope the sun is shining and everyone will be in good spirits." Box office 01491 843404 www.henley-festival.co.uk