There are some great fish in the Waikato system, particularly in the hydro lakes.
I don't have current info, but lived in the 'Tron for a few years and never had much luck in the urban parts of the river. It's quite funny when people who don't fish report seeing heaps of big trout under various bridges - they're actually schools of mullet.
I found the drive south to Arapuni very worthwhile, though - with very nice rainbows out of the tailraces at the top and bottom. They are (or used to be, anyway) stocked from the R type breeding programme.
Some experienced fly fishos used to get huge browns at night in Karapiro as well. Like the ones at Arapuni, they got big from scoffing the smelt in the system.
Search this Forum for Arapuni and you should find some threads with good tips. Due to the lack of casting room in many places, a good spin rod with a 1/4oz ball sinker and a trace to a smelt fly (my favourite was a sparkle green bodied Rabbit with prominent eyes) is usually less frustrating. Cast up and across and the fish usually hit it towards the end of the swing.
I did a few trips to the upper Waipa and other small streams over towards Matamata with fly but found the average size pretty small. If you're used to catching much bigger trout, the down scaling of gear and mindset wasn't my cup of tea. When hooked, the small rainbows often scoot all around and the hook damages their mouths and faces in a way that I found hard to justify.