Also looking for advice and/or a partner in May

Thank you very much guys! I'll look into it (aim for a month probably) and might ask more questions at a later time :D

@Yarmoss put it very well. My additions / reinforcements: For winter tramping a car will be a real help, if you can afford it. I'd expect to be able to get out tramping through most of May here in the south of the South Island, but would also expect to be planning around the weather - looking at next week's forecast and only then deciding where to go. Most days / weeks there will be somewhere with good tramping weather - east coast west coast, Central Otago in the rain-shadow of the Alps, Southland, Canterbury front faces, and ... if you're lucky ... Fiordland. But to try and chase a weather window on public transport would be more of a challenge. Snow? 'Snow in May, it won't stay' is the saying. So yes - snow is a real possibility, but given a month to play with you'd hope to be able to visit somewhere low-lying and tackle the higher stuff once it's cleared. Another vote for Caples-Greenstone from me. Dingleburn, or a return trip like the Huxley can be a good option when the main divide has bad weather. Sections of Te Araroa (http://www.teararoa.org.nz/) through the Canterbury front country give well marked track through much of that country (e.g. Two Thumb Range, Lagoon-Cass loop). Some of those are one-way trips (so maybe the car thing is not such a perfect solution) but others are loops. Te Araroa through the Richmond Ranges (Nelson) is another good option when other areas are rained / snowed out. And that's just the multi-day stuff - there are 100s of overnight options. And don't be shy to ask again when you're here and we can look at a forecast.

@Madpom, I'll be looking closely at the trails and ask again later. Thanks about the car, snow and so forth. Speaking of the later, I could still bring one of the following, which one would you recommend: a) Ice crampons like Grivel 12: https://www.mec.ca/en/product/5048-297/G12-Cramp-O-Matic-Crampons?org_text=grivel%20G12 b) Chains like: https://www.walmart.com/ip/Yaktrax-Chains-ICEtrekkers-Large/45259588 ? or c) Some microspike, but those are usually lower quality

the sandflies are bad all over fiordland and the west coast....

@waynowski Would they be worse on Milford than on Roeterburn though?

lower altitude is the worst

There are a dozen+ different species of NZ sandfly and the distribution varies throughout the country and with altitude. As a consequence, the altitude limit of their range varies widely. In the Milford/Routeburn area, they tend to diminish in voracity above 900m elevation and pretty much disappear above 1000m. Unless you're allergic to the bite (in which case you have a real problem), it's not that big an issue, imo. I use an 80% DEET topical repellent that works for me (just don't get any on plastics). If you're moving, they leave you alone. Yes, they are annoying - but the stunning scenery more than makes up for them. A few years ago, just north of the Routeburn and at 700m, I woke up to what I thought was rain. Nope, just sandflies trying to bash their way through the tent inner. 😂

we need a sandfly recipe book. That would save a lot of weight carrying protein sources

Actually 19 species but, apparently, only 3 bite humans (that's species, not individuals, sadly). At 0.04g, geeves, you'd need around 5000 to make a meal. Maybe a sandfly selective breeding program to replace the Moa :)

For maps you can download the whole topo map of NZ from here: https://sites.google.com/site/irnzmaps/ and use it with OruxMaps or Locus Maps apps on Android. This is absolutely great :) It takes a couple hours to try and get familiar with the app to see how to record your track, see your position, etc. but it is very very useful. Make sure to try your battery life (with the phone on airplane mode) to see how much you can use the app. For me when recording the track my phone lasts about 15-20 hours of continuous use (with the screen off but a few checks on the map from time to time), and about double or triple that if I don't record but just check out my position from time to time. Another good tip for electronics use is to keep the battery warm during the night if it's cold. Saves a lot of battery. Personally I put it in my sleeping bag or under my pillow.