The only access to the Pakuratahi that I could find was about 100m up the summit sidestream from the confluence - just downhill from a small cutting on the railtrail. A steep scramble through scrub and grassy slopes led to the small stream draining from Summit. A simple 100m walk down the flat riverbed / valley floor followed to the confluence.
The Pahuratahi is quite large and confined by steep valleysides and gorges for most of it's length upstream. The only spot I hit really deep pools (swimming required) was in the first gorge 300m upriver from the confluence. As such, I;d advise staying on the eastern bank and climbing over a small knob before returning to the river above the 1st gorge - at least I wish that's what I'd done!
Above that, the river is generally knee-to-waist deep in normal flows, and slow flowing, It would be impassible after heavy rain. Travel was for the most part on the stony riverbed with a few scrambles in thick fern & windfall vegetation on the banks for a few short pools & gorges. For the 1st 3km, campspots were frequent, but above that there was almost nothing flat enough, clear enough and big enough to pitch a tent until the headwaters.
As you head upriver, tight gorges become frequent, often blocked by logjams 5-10m tall become frequent. Above each is a flat expanse of gravel and easy going until you get into the next gorge, I remained almost continuously in the riverbed until the last main forks 3km below the headwaters. Going is very slow, I averaged 1.5km / hour, rather than my normal 2-3 for riverbeds.
From the last forks the river frequently becomes tight and choked - but the valleysides are now gentle and open enough to scramble along, and in places even walk! This section was about a 50:50 mix of riverbed walking and valleyside scrambles.
If you're watching your map, the point to leave the rver at the last forks and climb the ridge to the pass into the Waiorongomai is obvious - the now very small stream forking SW / SE.
Reasonable campspots exist on small bush flats between the forks. There are none heading down the Waiorongomai until pretty much the hut,.