When you find this cache, you will be about as close as practical to what used to be New Zealands largest airfield. Built in total secrecy in WW2, funded by the Yanks and designed to cater to heavy bombers, it was never actually used. The longest runway was 2 miles long and when I first had a gander ten years ago, it was very easy to see where it was and I could drive along parts of it. However, dairy farming has mostly wiped it off the map. The fuel depot at Bankside was also built to support this airfield.
At GZ, you can see a line of Wattle trees, it seems that Wattle seeds were in the gravel they were using and a few years later, wattles grew up in the runways and roads they were building.
Have a gander at Google Earth historical imagery from 2004 and you can see part of two of the shorter runways just south of the centre pivot.
You can read much more about it at:
http://en.wikipedia.org/wiki/RNZAF_Station_Te_Pirita
http://keteselwyn.peoplesnetworknz.info/places_of_interest/topics/show/616-te-pirita-secret-world-war-2-airfield
http://www.nzrifle.com/ml_canterbury_defence.htm
http://www.cambridgeairforce.org.nz/rnzaf%20stations%20south%20island.htm#TePirita
Anyhow, the cache is a 1.2l Sistema that shouldn't pose any problems.