I have actually had this issue occur to me also.
Simply way to fix it is get solar hot water!
Something I've heard is that a smart meter roll out may occur over the coming years which can and will be able to remotely change tariffs at different times of the day.
This could mean that whilst we are all using a lot of power on a hot day for air-cons etc, electricity companies could bump up the rate and potentially make more money in an attempt to lower peak loads in summer as it seems the power stations are unable to provide this sort of power (maybe stop brown outs also?).
If this smart meter does in fact get rolled out - you won't need to worry about off peak because it will not exist.
If anyone else has heard anything regarding this roll out please send a PM please!