I just started working with AusAID, so I might be able to shed a little light on how to 'get into' the industry. I use the phrase lightly because there really isn't a standard path to get into international development, and it certainly is nothing to do with your qualification. Nearly everyone in the industry has a degree in international relations/development/affairs, so these days you generally need to get a Master degree if you want to specialise. And you really should specialise, if you want to break into the industry. I'm an architect by profession and got into where I am now because of my specialist background.
The UN is extremely hard to get into. Each year they run an international competitive exam for graduates, called the YPP exam. I sat least year's exam (they happened to be looking for architects), 45,000+ people applied, around 5000+ got to sit the exam, around 200 are getting interviewed and less than 100 people will be offered jobs. 100 out of 45,000. If you are one of the lucky few that get a job in the UN, be prepared to move around the posts every 2 years, and to not have much stability in your life.
If you really are interested in getting into the development sector, then a B of international development is fine to start off with, but I would recommend finding a niche specialty, doing volunteer work overseas with NGOs, and applying for AYAD programs (AusAID funded volunteer programs) once you get some experience.