Melbourne-based explosives manufacturer Orica has secured supply contracts in the Pilbara with BHP Billiton and Roy Hill, but expects operational issues and write-downs to cut its first-half earnings by nearly $400 million.

Melbourne-based explosives manufacturer Orica has secured supply contracts in the Pilbara with BHP Billiton and Roy Hill, but expects operational issues and write-downs to cut its first-half earnings by nearly $400 million.