Duff,
From memory, you aren't required to pay back a NZ student loan until you are earning above a certain threshold, last I heard was about $NZ15K p.a. Unless you are employed on salary as a flight instructor I would be surprised if you breached that threshold as a new C-cat on flying pay. Maybe as a senior B-Cat Multi you would get there!
As for leaving the country, I heard that the Government has started making student loaners declare arrangements for repaying a loan whilst o/seas regardless if you are working or not. I'm not sure but check it out. Those supplying a student loan should have the information.
On the other hand many blokes and gals just get the loan and qualifications and worry about it afterwards. You've got to be in to win and to be in you need that basic quals + experience.
Who are planning to do this diploma through??
S2K