Welcome to the NZ Forum. These are probably the three most popular of the Great Walks so well served by public transport in the peak season. I assume you will coming next summer, rather than attempting these in the winter months? Which NZ city are you starting from?

ATNP- you have to get to the city of Nelson first by bus or air. Water taxis leave from the nearby beaches of Kaiteriteri & Marahau & I'm not sure who runs the connections between. You can do any length of walk from hours to about 5 days as the water taxi will pick you up again if you wish.

The Milford & Kepler tracks both begin in the town of Te Anau, which doesn't have an airport so you need to fly or bus to Queenstown, then on to Te Anau. Flights from Nelson would usually route through Christchurch as would a bus. The bus route would take a minimum of 2 days, but the alternative route via the West Coast glaciers could take 3-5 days, depending on what sightseeing you wanted to do enroute.

The Kepler is a loop which can be commenced at the DoC office in the town. You need 2 or 3 nights on the track.

The Milford Track is the most tightly regulated - you must stay 1 night in each hut & must commence at Te Anau. You need to book the boat ride across the lake at the same time as booking your huts. I assume you can get a shuttle bus to the jetty. At the end of the Track, you need to book a boat out from Sandfly Point to Milford Sound and then any number of coaches will take you back to Te Anau or on to Queenstown.

You must book the huts early esp for Milford as there is a limit of 40 trampers per day.