Not so sure about how/when it came to NZ, but here's some information about how it came to be in the US. Thanks to my husband, who found this link because A) He loves me and B) He loves to be right. :-)
I always thought the 40 hr week came about in Britain late 1800s when they developed production work - assembly lines - they found that workers were at their best productive level in four hour increments with break for lunch - and after 8 hrs of assembly work they started making mistakes - tired, etc. so they had to hire more workers and double shift them in order to get the most out of their machinery. It makes sense. Most workers don't do that well when they work longer than 8 hours at anything during a day - so if you're working 60 hour weeks you may be actually wasting a lot of time and ruining your body for no real gain (unless you're paid $$$$$ per hour and the boss doesn't realize you're sleeping on the job!) Me, I'm on call 24 hours a day, so it makes no sense for me to worry about it. I don't work very hard but there is a psychological burden having to be "available" all the time, for sure.