According to the Wikipedia, the fifth generation Chevrolet Camaro is using the GM Zeta plaform which it is the origin of Australia. This site has been using USA as origin and some are build in Canada. Should this be Origin of Australia for the platform or USA for the body?

http://en.wikipedia.org/wiki/Chevrolet_Camaro_(fifth_generation)

It is only the platform which is Australian, not the whole car.
They always share platform between many cars of a group.

Hard to say. Sure the platform is from Australia, but we don't usually just go by platform for origin. For the Camaro a lot of design and engineering aspects were probably shared by both GM USA & Holden teams.

It surely makes a lot of sense

Sounds like this model is intended for the USA market.