Permits , bookings and camping or hut fees should be paid via DOC in NZ, preferably in advance during the peak walking season. Huts and campsites can be booked out.
During the walking seasons there are rangers in residence along the way to manage the tracks, check permits and provide assistance if required.

Maps and information specific to each of these walks can be obtained from DOC in NZ.

If these two walks are to be combined one scenario that's been tried successfully is as follows. All accommodation and transport needs to be pre-booked, preferably before arriving in NZ at peak times. Enquire to be sure.

Start - Queenstown NZ. Pre book for huts or camping. Book a local company bus transport (eg Tracknet) to the start of the Routeburn Track at the Routeburn Shelter. Follow the 3 day, 2 night itinerary (ie Routeburn Flats, Lake McKenzie) to The Divide Shelter.
From The Divide pick up your pre-booked transport to Te Anau for a night's stopover (at least) and to restock.
From Te Anau either walk/ride around the lake to Brod Bay or take a boat across the lake to Brod Bay.
Follow the 4 day 3 night itinerary ( Luxmore Hut, Iris Burn Hut, Moturau Hut) to Rainbow beach. From there either walk or take booked transport back to Te Anau.

Everywhere, but still carry at least a litre.

NZ parks are tightly managed. Permits are required and, often, checked. No gates, no fires needed. The huts all provide water and cooking facilities - just bring your own food and cooking and eating utensils.
If you take the camping option (ie on the Routeburn only) you will need a cooker and fuel as well as your tent.
Note: There are strict quarantine restrictions on entry to NZ. Only commercially packed and sealed foodstuffs are allowed and no dirt or soil is tolerated in gear. Therefore any hiking food you've prepared/dehydrated yourself may be confiscated and boots may be inspected and tents unpackaged and cleaned on arrival.
Previous visitors have found it easier to buy food in NZ. There are good supplies of hiking food and the NZ made dehydrated meals are reasonably priced there. Check current the rules before departure.

Both routes are described in information that is readily available and the tracks are well defined and well made. However weather conditions can change dramatically and damage or ‘white out’ tracks so be prepared. Strong winds can also occur.

Both tracks have shelters between the main residential huts. The Kepler provides the highest accommodation in NZ at Luxmore hut.  You can get snow there at the best of times.

There is no serious altitude involved in either walk.  They are not hard and do not involve climbing or exposure. However they can be cold, wet and slippery and even if you are not carrying a tent you should have adequate warm and waterproof clothing and snack food  to stay put out side for a night in an emergency.

The rangers will close the track if they think conditions have become too hazardous.

Routeburn – Designated tent camp sites available as well as the option of huts – both need to be booked.

Kepler – Serviced, pre booked huts only.  Shared dormitories with mattresses.  BYO food, cooking utensils and sleeping bag.

This entry is just a guideline to these two well known NZ ‘Great Walks’ which despite their popularity and, therefore, fellow travelers, are very well worth doing.  The Milford Track is another well know, hut based Great Walk’ not discussed here.  There are many other walks in NZ, not all as well serviced and used as the ‘Great Walks’, but still very popular and spectacular.

eg:  Mt Aspiring National Park and Rees & Dart River tracks. These need to be booked but are entirely DIY in all respects but with huts and marked trails.  However a good map is recommended in these cases.

Best (and most popular) time of year weather-wise is probably February but conditions can vary dramatically.