The model, who is mother to 2-year-old son Flynn with husband Orlando Bloom, has worked for the lingerie brand for three years.
She explained to The Sydney Morning Herald: "This is a natural evolution. The thing is, I've been modelling 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 realised I needed to prioritise my time.
"I'm just not in a position to commit to a full contract at this time. It's three months at a bare minimum."