The Alps 2 Ocean Cycle Trail, the A20, is a 301 kilometre (187 mile) ride from the base of New Zealand's highest mountain to the coast.
It's not actually a trail... It's mainly a set of signposts that guide you along roads that are quiet and scenic. A small part of the total ride is on purpose built track. Expect to be wowed by alpine vistas, expansive high country basins, numerous lakes, canals, history, quaint towns and finally the blue Pacific Ocean.
The ride is popular with recreational cyclists who like multi-day supported tours. It is typically ridden over 4 or 5 days. There are reasonable food and accommodation options along the way and plenty of tour operators to organise everything and carry your overnight gear. It's the kind of ride you do with friends or family as a mellow summer holiday adventure.
The trail has an excellent website with suggested itineraries, services, trail maps, lists of must-see attractions, video, and much more including photos of the pretty girls you'll meet along the way. You'll find it here: http://www.alps2ocean.com/ or alternatively google "Alps2Ocean" for oodles of commercial sites offering services. At least a dozen tour operators can help with bookings, bike hire, shuttles, and so on.
Some sections of the trail make reasonable out-and-back day rides. A couple of those are described here:
http://www.singletracks.com/bike-trails/tasman-river-track.html and here: http://www.singletracks.com/bike-trails/tarnbrae-track.html