Hi Alex,
I don't know what you are planning on doing for accommodation.
But thought you may not know that "freedom camping" is becoming quite unpopular with regional councils in many parts of NZ.
Many have by laws and quite steep fines if you get caught.
In my neck of the woods the Christchurch City Council is looking at bringing in by laws somewhere around March April.
As per a small group stuffing it up for the majority.
Your 2 longest trips are a breeze

But you will want to factor in time on the ChCh Gmouth one for a few hrs spent in the Arthurs Pass area.
Plenty of shortish bush walks in the area.
The Devils Punchbowl waterfall being a nice leg stretcher, a chance to see some nice scenery and do a bit of birding.
The same goes for Inv to Dn, the Catlins are well worth getting waylaid in.
This is NZ's main birding site.
http://www.birdingnz.net/forum/
Plenty of friendly Kiwi birders to give a help with bird spots.
Have a look in the general discussion area, several threads on traveling and birding here.