I don’t think I have ever seen anyone actually fly fishing in the Twizel Canals. I have seen the odd youtube video of fly fishing there, but in all my trips to the canals I have never actually seen anyone using a fly rod. I do know of at least one bloke from Australia who has been successful with the fly rod though. I’m sure he used plenty of weight to get his flies down deep. Here’s the link: https://fishingmag.co.nz/forums/topic/twizel-canal-fish
He said he used a little split-shot for extra depth. There is no reason the fly rod shouldn’t work well especially in the Tekapo and Ohau A canals which are a bit shallower and not as wide as Ohau B and C. Casting a nymph in the white-water at one of the inlets would be sure to work especially as the days get colder. April should be all good. Drifting a couple of nymphs along the wall would also be the go but you would have one hell of a fight on your hands with an angry 25lb rainbow jack on the other end. Good luck!