I've been playing around with half hour GXP data provided by the Electricity Authority (which replaced the Electricity Commission in November 2010). Thank you EA! You can visit their site here: http://www.ea.govt.nz.
The data came with a disclaimer that they can't guarantee it's 100% correct - and I could also have made errors when processing it. [Update: 27 April - I DID make an error and the first version of the graphs that I posted had demand value labels that were all double what they should have been. Sorry about that - all fixed now! The "shape" was correct - just the labelling on the y-axes was wrong.]
Bearing in mind those caveats (I make no guarantees whatsoever - this is provided just for interest), here in graphical form are some of TLC's highest three hour "kW load" values from Ohakune. Note the values are actually megawatts (MW), 1000 times larger than the usual kilowatts (kW) that apply to domestic installations and businesses.
Not surprisingly, these three peaks are all in the July school holidays so no doubt Ohakune had a few extras in town over that period.
The Electricity Authority also provide a web interface to some of their historical data (which they call the Central Dataset, or CDS). That's also pretty useful if you're don't need the detailed data. The web site version is aggregated to day or month for example, while the more detailed half hour data comes on a DVD and needs quite a bit of effort (plus the right kind of background/skills) to make much sense of.
[27 April 2011: For the more adventurous, half hour demand data (by Grid Exit Point such as Ohakune - code OKN0111) is also available from the "demand" section of the WITS "Free to Air" site. This data is updated daily and doesn't require sending away to the Electricity Authority for a DVD, although it doesn't show how the offtake from Ohakune is split between TLC and PowerCo, so is slightly less useful in that respect.]