For NZ only:
the best publication to reach the widest railfan readership is "NZ Railfan".
published 4 times a year, 80 A4 pages on high quality heavy paper.
They don't have website (but you can buy it on other websites)
The email contact is: Mc0@xtra.co.nz
who will be able to give you rates.
You will have missed the September issue. The next one will be in December.
The other magazine is "NZ Railway Observer". published 6 times a year (much less pages). I don't know what the readership levels are, but I would have though it much, much lower than the NZ Railfan.
Their website is: http://www.railsoc.org.nz/ro/observer.htm?Observer%20Index%20Page
We do get some of the Australian mags over here, but I have never looked at any so I couldn't tell you want one to try if you are trying to cover both countries in one hit.