Hi Dasha,
I'm a six year med student also at Auckland, but originally from the states. Feel free to PM me for any advice about post-graduation. If you want the quickest way to becoming specialized the US is the number one choice, but I would say you really need to think about where you want to end up one day. If it's NZ or Australia your best bet would be to stay in NZ cause you'd have a much easier time getting on a training scheme cause you went here for school and then you could always move to OZ once you were a consultant. If it's back to North America then you need to take the USMLEs (preferably taking Step 1 in Fifth year or just before 6th year starts then take 1st part of Step 2 early in 6th year and second part later in the year (this part has to be taken in the US) ). I would also advize you to do your 5th year elective (4 weeks) in the states (I did mine in New Orleans, LSU program cause they don't require you to be in your final year like most US programs do). Ideally you should also do your 6th year elective (8weeks) in the States as well. The reason being this way you get some American Letters of Recommendation plus you can take the Step 2 CS exam while you are over there. From a few of my Canadian classmates they tell me the American scheme is a lot easier to get into as an IMG then the Canadian one.
If it's Britain you want it will probably be extremely hard to get on a training program. That's why a lot of the registrars you meet while in the hospitals here will actually be British. It's nearly impossible these days to get on british schemes without an EU passport.
Ireland is also a longer process but if you work one year as a house officer in New Zealand and get Australiasian Registration then you will also currently qualify for full registration in Ireland as well (this might be changing soon). Not too sure how hard it is to get on a training program there however.
As far as pay, lifestyle, etc for the different countries go. I think New Zealand and OZ probably have the best balance. This however is only my oppinion but it is one I have deduced after taking to many registrars from all of these places.