The offtake agreement with the New Zealand agricultural company is the third such deal Centrex has signed to underpin sales of phosphate from its Ardmore rock phosphate project in Queensland.
Centrex also signed a marketing agreement with Samsung C&T in February for sales of product into Korea, Japan, Indonesia, India, Mexico and Taiwan.
In March it signed a binding offtake agreement with Ameropa for the supply of Ardmore rock phosphate.
Centrex managing director Robert Mencel said by Centex was fulling its offtake plan by signing the offtake deal with Ravensdown.
"This agreement also continues our expansion of sales of product into the lucrative New Zealand agricultural market," he said.
"Similar to our other offtake partners, we look forward to along-term mutually beneficial relationship with Ravensdown."
Centrex sent a trial shipment to Ravensdown last year.
It has set aside 20% of Ardmore's production to fulfill its projected allocation under the agreement.
The supply of phosphate rock under the agreement was due to begin on July 1.