Omnimap seem to have all 1:50k maps with a delivery time of around two weeks.
However, Omnimap are quite expensive (US$ 16.95 per sheet).
By comparison, MapWorld New Zealand charges NZ$ 7.90 (US$ 6.50) per map.
Land Information New Zealand have a (probably incomplete) list of international resellers .
This list includes Omnimap.
Probably some of the domestic resellers also sell to foreign customers, so they might be your best bet.
outdoors.stackexchange.com/users/566/gerrit
I am from New Zealand
and I tend to agree with the topomap service.
Another great resource is the Department of Conservation Website http://www.doc.govt.nz Happy Hiking
outdoors.stackexchange.com/users/1877/Agit
I'm a kiwi too (a director of Hiking New Zealand.com)
We do remote guided small group trips around NZ.
Some of us are starting to play around with digital map solutions.
A good one I found is BackCountryNavigator PRO.
It cost a few bucks but is excellent.
You can download maps for when you are out of Internet range and your GPS works on it.
Don't use Google Maps or Apple Maps for wilderness info -- tracks etc are unreliably marked or out-of-date.
excellent otherwise tho
With this tool you can do all your research at home and just buy the odd map as a back-up or for the more full on trips where you really can't afford a smart phone 'incident'.
Good luck and enjoy our wonderful back-country.
If you want take a look at our web site - we have a few hiking tips and ideas there too.
Reece dart is nice, plenty of other options, try and stay away from the crowded great walks huts if you are looking for solitude :-)
outdoors.stackexchange.com/users/1878/malcolm
Quick tips: Go to http://www.i-needtoknow.com/milford/maps/index.html
They have several links for hikers, including maps.
One
you might check out is the Department of Conservation's online maps at: http://www.doc.govt.nz/parks-and-recreation/tracks-and-walks/
Also, Google says there's another site called www.kiwifootpaths.com that may be useful...
outdoors.stackexchange.com/users/1870/Paul Zimmerli
NZ Topo Map - www.topomap.co.nz is a great resource.
You can highlight the area of map you're interested in and print it out in high quality to take with you.
If you have a Windows Phone then you can download the NZ Topo Map app and take sections offline with you.
Or if you have an Android phone, New Zealand Topo Maps Pro by Atlogis is a great app for taking sections of map offline with you also.
outdoors.stackexchange.com/users/2596/Gavin
You can browse the 1:50,000 Topographic Map of New Zealand here: https://data.linz.govt.nz/layer/767-nz-topo50-maps/ You can download it too
but: a) you need to register with a valid email address and create a password b)
the whole thing is about 10GB
and you can only download 2GB at at time
Just set a crop around the area you're interested in and you can download the maps for that area.
Depending on where the boundaries of the crop are you will get some entire sheets and some portions of others (see this index ).
The images are georeferenced and you can chose which projection/datum when you download.
Definitely get paper copies before you hit the trail but it doesn't hurt to study the digital versions beforehand.
outdoors.stackexchange.com/users/9431/ghosteditor
As of November 2015, Google Maps now has StreetView for the following tracks : Milford Track Kepler
Track Abel Tasman Coast Track Lake Waikaremoana Heaphy Track Routeburn Track Rakiura Track (Stewart Island)
While the purists may bemoan this, they can be a good resource for identifying challenging parts, highlights or becoming familiar with a route.
As always, StreetView is never as good as actually being there.
outdoors.stackexchange.com/users/9405/timbo
These are sort of a secret tip i got and took us on many incredibly awesome hikes: http://www.nzfrenzy.com/
My girlfriend and I travelled through NZ for a couple of months and mainly used these as a foundation for our trips.
We were very glad to have them and recommended them to everyone we met.
I was actually hestitating on sharing these because we enjoyed the fact that many of the places arent well known and not overrun by tourists.
It also features well known highlights though, you wont miss out on any NZ awesomeness.
outdoors.stackexchange.com/users/15068/Ostkontentitan
Me and my girlfriend will go there in January and we plan on hiking for a week or so. Unfortunately there are no maps or books available for NZ hiking in my area. Could you recommend some for me?  Also I started using a GPS recently so the maps can be in any form (digital or paper), but I'd prefer to have a paper version as well.  We're looking at Rees Dart, but we're opened to any recommendations you might give us.
New Zealand recommended hiking maps, books and other resources
outdoors.stackexchange.com
<hiking><maps><trails><new-zealand>
outdoors.stackexchange.com/questions/3133
outdoors.stackexchange.com/users/1860/Paul Irofti
Recommendations and tips include: http://www.nzfrenzy.com; http://www.i-needtoknow.com/milford/maps/index.html; BackCountryNavigator PRO; www.kiwifootpaths.com; www.topomap.co.nz; and Omnimap. Google Maps or Apple maps are said to be unreliably marked or out of date. Official websites include the 1:50,000 Topographic Map of New Zealand here:  https://data.linz.govt.nz/layer/767-nz-topo50-map; and the Department of Conservation's online maps at:  http://www.doc.govt.nz/parks-and-recreation/tracks-and-walks. The New Zealand topographic map and DOC site are useful. Topo, Omnimap and BackCountryNavigator Pro are helpful too.
Topo, Omnimap, BackCountryNavigator are useful resources. The New Zealand Topographic map and DOC website are good resources.