Hi Kyle,
As max42mtb has said, the Facebook group he listed is definitely the best resource for anything in NZ.
I can give you the following info though from a trip that 3 of us (Aussies) did last January.
We camped at the Kowai Pass Reserve campground on the edge of Springfield then left our one and only hire car there. Its a 1.5 km walk from there to the Springfield Train Station where we boarded the TranzAlpine train to Cass.
We had lots of rain so only had to walk 1.5km from Cass to the Cass River. From there we were able to paddle/float down it 3.5km to the Waimak. Normally though there would not be enough water for this and instead you would walk (or perhaps try to hitch) 3.5 km to the Mt White Bridge. This is the normal put in location.
We paddled down to Hamilton Hut and stayed the night there before continuing the next day. We got to our pull out point at Keens Rd / Kowai River junction around the middle of day 2. From there it was a 6km or so jog back to the car. The managers of the reserve were really good about us leaving our car there but you could also leave it at the Springfield Station.
If you haven’t already, use http://www.topomap.co.nz/ for easy access to detailed online maps of the whole of NZ.
I think its a great loop for those relatively new to packrafting (depnding on the river level and the group of course).
Enjoy your trip!