Christchurch Airport Customs?

This sounds more ike coincidence thank the norm. Customs have the right to detain anyone that they think does not have a genuine reason for visiting, and this is no different across the world. Some reasons could be that they may think you are entering to gain work and overstay your visa period. The levels of people doing this could be greater in New Zealand than other countries you have visited, and their checks greater and take longer, I can't comment on yours or the other cases, as you give no details.