Good write up from Benjamin, it is slightly (only slightly different) in NZ. Unfortunately this is a topic that no one other than the maxfac's themselves seem to know a lot about, but from what I gather the timeline goes like this:
Graduation from BDS -> work preferably 2 years at a DHB (does not need to be a tertiary institution but there are probably some which are more preferable to others ->, sit your primaries for the FRACDS in your second year of working -> entry into 4th year of MBChB -> do PGY1 and PGY2 after graduation -> entry into the OMFS programme - so to answer your question olives, no you are not guaranteed entry into the OMFS programme. Note that that is the ideal as well, you may have to do more years after graduation from BDS/MBChB.
Your GPA is probably important but not the most important; also if OMFS is similar to other dental specialties then research is very important so I would encourage you to get involved with Summer Studentships and perhaps even Honors, and do a really good job on your 5th year elective - try to get published. I would also agree with Ben that relationships are really important.
As you can see, OMFS is a really arduous process and I would imagine only 1-2 people get accepted every year in NZ, if even that. There are other pathways if you are interested:
1) there is an OMFS equivalent where you are guaranteed entry into the programme before you do MBChB, but it is recognised in NZ only.
2) if you're interested in doing surgery, then consider doing your DClinDent in Oral Surgery, which does not require you to do medicine. There's a lot of overlap between the two, you can do orthognathic surgery, place implants, treat oral cancers etc etc but you are limited to from a simplified explanation, the lower half of the face.
The nice thing about doing dentistry first is that you will be able to work part-time in the weekends, but do consider the many many years of hard work and also the cost of training.
Venronux may be able to correct me on things.
PS as a side note, please use the 'edit' feature rather than double- or triple-posting ^_^