Firstly you do not get to buy spot power prices as anything short of an industrial user in NZ. I used to administer one of the systems for submitting bids, so unless you're a milk factory or aluminium smelter, you're buying at retail.
Pre-buying retail power (powershop? Unless they've gone under) is perfectly fine, but like almost all power retailers they buy at wholesale, usually in advance (ie not spot), then sell to the plebs at retail. Powershop and the ilk let you buy in advance, and pass some savings on to you. However, when I was last living in NZ there was almost no point in shopping around for power prices. I've always had gas, and there are usually only one or two companies that supply gas, and the discount they give you for a combined tarif is lower than any savings that can be made from pre-buying power, or the cheapest supply options.
The only way you're "saving large" buy pre-paying is because you where on a rip-off tariff before. But that's pretty much the norm for NZ utilities. Don't get me started on the fucking phone lines. You should save roughly 20-30% if you go from a "pre-pay, bad credit" to a "pre-pay". Most would give me 10-15% off for simply paying on time.
A power audit can certainly help people cut down on their bills. My experience of NZ houses and UK flats is that about two thirds have some sort of "power leakage" where if you turn off everything, you still are chunking through the power. NZ ones mainly shitty wiring and some power theft, UK ones mainly power theft with some shitty wiring. But once you've worked out what optional appliances can be switched off, and defrosted the freezer etc, then you can't do much to change your usage. There's a limit to what you can turn off, and what appliances you are willing to replace. Smart meter won't save you anything by itself, only a change in behavior.
Mind you, there are times in the past when I'd like to have had a smart meter. Mainly when the previous person lied about the meter reading to the power company (who accepted that for 12 months he used ~2 months worth of power) and then the power company tried to charge us for ~18 months of power for 6 months of usage during the warm part of the year.