I need to have english documents (birth certificate, university diplomas....) translated in chinese.

You can translate the two documents at a local Notary Public, then you need to follow the below procedure:
Step 1: Each document should be notarized by a local Notary Public.

Step 2: The notarized document should be then authenticated by the New Zealand Department of Internal Affairs.

Step 3: The document should be then authenticated by the Consular Department of the Ministry of Foreign Affairs and Trade of New Zealand.

Step 4: The Chinese Embassy authenticates the signature of the authentication officer and the seal of the Consular Department of MFAT.

You need to fill in an application form for authentication either at the Embassy or download it from website www.chinaembassy.org.nz. The copy of the identify page of your valid passport is required. the embassy may require some additional documents in order to complete the authentication.