Back in 2013, I joined for one month a couple of friends hiking the Te Araroa Trail southbound. We met close to the trail head of the Whanganui River where you basically need to rent canoes in order to go down the river until you reach the ocean close to the town of Whanganui. The river was relatively calm in this section, but should never be underestimated since heavy rain can let grow quickly. (Further upstream, you can obviously find nice spots for packrafting.)
We then headed to the south island in order to continue the trail on the Nelson Marlborough part before heading into the mountains following the Richmond Alpine Track.
There are lots of information on the web about the Te Araroa Trail in general – I can specifically recommend teararoa.org.nz with its very helpful FAQ as well as its comprehensive download section where you can find plenty of PDF maps, KMZ files, gpx files etc.
New Zealand, specifically the south island for me, is definitively a place I need to head back to.