Another view - I doubt that Ashburton's thunder incidence is significantly different to Christchurch's, or any other place on the Plains. But I agree to lump all of Canterbury in as a low thunder zone is a bit misleading - there are known favoured generating sites for convective (summer) storms eg Mt Peel area, Mt Hutt range, Hanmer Basin which probably have a much higher incidence of thunder than Christchurch or Ashburton or Rangiora or most parts of the Plains.
However, Steven is correct in that the topography of Canterbury does block the arrival what we might call advective thunderstorms in unstable air masses. The only directions open to the ocean are from Northeast to Southsouthwest. Airflows from northeast to Southeast are rarely unstable enough to support deep convection, while flows from SSW to SSE are usually too dry due to the very cold surface airmass. Flow from any other direction is blocked by topography.
So, the two main types of storm we see are those generated on (with) a cold SW change as it moves up over the Plains, and as we have seen in the last 3 weeks, only a very small proportion of such changes have the right combination of factors to generate storms; and true convective storms, which mostly develop on the inland high country, and blow across the Plains in unsteable westerly quarter airflow. And they need the levels of solar radiation to generate big enough thermals that we generally only see in the summer months.
However, there are storms, and there are storms. One of the nice things about Canterbury summer storms is that, although they are relatively rare, they are usually easy to see from a long distance away, they look spectacular, and they often produce the classic thunderstorm weather at the surface, unlike the many storms which more regularly affect western districts, but seem more like thundery showers, quickly passing by, than 'enjoyable' storms.
Feel free to disagree
) - I was going to say something similar. Many thunderstorms in northern and western places (such as Auckland) occur in rainy situations where there's usually alot of heavy cloud cover, so the thunder and lightning may be spectacular itself, but one misses seeing the visual splendour of the cb tops. Down here, the storms, even though they're rarer, are visually more interesting as they can be seen from a distance. (though spectacular 'thunderheads' can sometimes be seen in Auckland and other places as well - as shown on some photos in this forum)
which makes my eyes run, sneezing up to 17 times in a row 
