Turns out my hot water system is fine but for some reason the relay in the meter box has stopped working. The electrician bypassed it, by hooking the hot water up to the "normal" supply rather than the off-peak, and he's sending through a request to the electricity comm to replace that relay when they come out to add their new meter for the solar.
I now have lovely, lovely hot water again. And the sparky pointed out how, prior to him rerouting my hot water system, my main meter was spinning backwards. That, he explained, was a result of the solar panels generating more electricity that what I was using. And that's what will be feeding back into the grid at a rate of 44 cents per kw/h as soon as Energex get their meter in place.