How codeshare revenue is split is a commercial arrangement between the partners with no single model. See this article for a discussion of the issues and some of the revenue sharing methods:

http://www.prosrm.com/articles/PROS_..._alliances.htm

One thing to remember, although it hopefully has no significance, is that under CTA rules, the tariffs applicable to the entire flight are those of the ticketing air carrier (i.e. fly AC flight code #, AC tariff applies; fly NZ flight code #, NZ tariff applies) regardless of whose metal it is.