I have fished the Ngongi a fair bit, and although now based in Auckland I was lucky enough to have a few uni placements in Rotorua meaning I could fish multiple times each week.
As others have said the public access areas are very much hit and miss. I have had some great days on those public access points, I have seen many fish above 8lb come out of those areas, and have twice been with a fishing mate who has maxed out his weigh net. All of the hogs that get into the private areas upstream go through that public access too, you just have to be there at the right time, and find one that is prepared to eat.
Then there are other days where it just sucks. You can't find a half decent fish to save yourself.
Like any river it does change, and where the fish are holding seems to change. Surprisingly enough the best "luck" I had was when I was based there for 3 months, rather than driving down from Auckland on the occasional weekend when Im not working and the weather looks reasonable.
I guess its like any river- the Tongariro and TT do not produce on every trip- but at least on those rivers when they don't produce they are pretty to look at, the Ngongotaha is really just a ditch which runs through what is far from the most scenic town in NZ.