Here are some useful links to help you find out more about soldiers who fought in WW1:
Auckland Museum Online Cenotaph: Great to start research on a relative, simply enter their name in the Person Search and you will have access to basic military records. http://www.aucklandmuseum.com/war-memorial/online-cenotaph
Archives New Zealand: For more detailed military records of soldiers.
WW100: Wonderful website with many features. Provides an even more comprehensive list of sources and sites to find information about those who served during WW1. http://ww100.govt.nz/sites-sources/
NZ History: Has a wealth of information on WW1 including this guide on how to conduct an investigation on First World War soldiers. https://nzhistory.govt.nz/war/researching-first-world-war-soldiers
Commonwealth War Graves Commission: This is a great website to easily locate the graves of New Zealanders overseas. http://www.cwgc.org
Have you found any other useful websites? Post the links below.