Tianjin Airlines is offering return airfares from Auckland to Xi'an for $473 return. Qantas is offering return airfares to Beijing for $665. This is $20 lower than the last time this Qantas fare was offered.
Have been to both Beijing and Xi'an. Highly recommending seeing the Terracotta Warriors in Xi'an.
For the Xi'an flight, it seems to connect in Chongqing. Some flights connect with 2 and have long layovers but there are loads of options. Make sure to pick the one with the fewest connections and layover time. (Used June 19-25 as sample dates).
For the Beijing flight, there is a short layover in Sydney. Qantas is a full service airline so you get food and bags included vs. Air Asia where those are extra. Also Qantas flight is ~19 hours with 1 stopover while Air Asia is ~30 hours + 2 stopvers. So $665 for Qantas vs. $520 + $199 for 1 checked bag/both ways, total $716 for Airasia.
Use Qantas' website and search for flexible fares to see where the cheap dates are. Screenshot
I've been to both as well. Xi'an is a very traditional Chinese city. Speaking a little Chinese will go a long way as it's not as tourist friendly as Beijing (which isn't saying much)..