it's possible to get a job offer before you start your application, but it's not the easisest to acheive. A lot depends on whether you work in an area of serious shortage - nursing, IT are a couple of examples...
the internet is your friend in terms of looking to see what jobs are available in NZ and having started your application already will show that you are committed to the move at least, which will be a bonus when speaking to recruiters/companies
also, yes people with job offers attached to their EOI applications get fast tracked (and have 50 more points too)