1. make sure the marriage certificate is not a 'copy of particulars of marriage'. if it is then you need bring your copy of particular to internal affair and cost you around $30 to get a marriage certificate from internal affair.
2. post the marriage certificate to Wellington address as :
The Authentication Unit
Po Box 805 Wellington
Level 13 Prime Property Tower
86-90 Lambton Quay
a free call is 0800872675
3. after get the verification from Wellington then you can bring those things to Chinese Embassy.