Doesn't Consumer or someone have a web site where it calculates the cheapest option for you, and with which company?
I've never heard of Powershop. We have been with the local company, Trustpower, forever and never had any problems so stay there. Our bills are estimated one month then read the next. Bills arrive online with plenty of time to pay, or dispute the amount, and then the total is magically sucked out on the specified day. On the rare occasion I have rung the helpline with a question, it has been answered promptly and helpfully by someone local who has answered my question efficiently.
I'm not actually sure what other options we have here :-?