Don't worry too much - I got mine the day after I arrived from the UK by visiting the IRD office - they are very helpful here!

If you look at the IRD website they say

"Complete this form to get an individual IRD number.

Send in a photocopy of one of the following documents as evidence of identity:

Birth certificate
Current New Zealand passport
Current New Zealand 18+ card
Current overseas passport
Certificate of New Zealand citizenship
Current New Zealand photo ID driver licence
A Certificate of Identity issued by the New Zealand Immigration Service"


So you should be able to get one in advance, although as I said they are so efficient here it may not be worth the effort - unless you are going to be earning money here before you arrive!

http://www.ird.govt.nz/library/publi...irdnumber.html

for more info

Hope this helps