"This is a natural evolution," Kerr, 29, told The Sydney Morning Herald.
"The thing is, I've been modeling since I was 13. I'm now entering a new phase in my life. I have felt this coming since my son was born and, after I became a mother, I realized I needed to prioritize my time," she said. "I'm just not in a position to commit to a full contract at this time. It's three months at a bare minimum."
Kerr is married to actor Orlando Bloom. They have a 2-year-old son together.