She told London newspaper The Daily Telegraph that the pressure of living in the public eye and keeping to an intense performance schedule led to a breakdown in early 2009.
Westenra, who has sold more than 2.8 million records and performed with some of the world's leading orchestras, told the newspaper her unhealthy eating habits became self-destructive.
The danger signs were present several years ago when she started to obsess over what she ate.
"Initially I thought it was healthy. I wouldn't have dairy, I wouldn't have sugar. When you reach about 18 you stop being able to eat what you want. I saw a picture and I thought I looked full of puppy fat," the 24-year old singer said.
Westenra, who signed with Universal Music at the age of 12 and moved to London at 18, found she was away from friends and family and touring non-stop.
"Food was a thing to focus on. I got into the habit of not eating much."
Towards the end of 2008 Westenra contracted bronchitis and, after trying to sing through it, finally decided she had to cancel the remaining dates of her tour.
She returned to New Zealand for Christmas and, as soon as she got home, she was hit by an "enormous wave of exhaustion".
"I got really depressed. I convinced myself that I could never go back.
"Usually if I'm ill, I eat healthy food, but [instead] I thought, 'I don't want to go back on stage, I want to eat ice cream and chocolate biscuits'.
"It was as if I didn't want to get better, I didn't want to be able to perform. I became self-destructive and it was a vicious cycle.
"You're tired, you eat really bad food, you feel worse, then you eat more. I wasn't sleeping."
The death of two of her friends in road accidents within weeks of each other -- one in New Zealand, one in Croatia -- let her to re-evaluate her own life and remember how much she liked it.
"I rediscovered my passion for performance and realised how important singing was for me," the soprano said.
Her fifth album, recorded with Ennio Morricone, will be released on August 29.
