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I was recently told while purchasing antifreeze at SuperCheap Auto, that as I lived in the south island, 30% mix (-18C protection) is not good enough and that they were obliged to inform me that I needed 50% mix (-37C protection). It was hand wavingly explained to be to do with wind chill effects.
I have a bit of a physics background, and I think this is complete bunk.
Especially since I live in Christchurch, I'm highly unlikely to see -18C air temp. Also while moving, (presumably the point where wind chill comes in) my engine will be heating the water quite thoroughly (coasting downhill may be the exception).

Can you confirm my logic is sound, and that the advice form an auto store is rubbish?

Coolant additive is not only put into the system for its anti-freeze qualities. It is also a corrosion inhibitor, lubricant and acts to raise the boiling point too, so there are a number of reasons why some car manufacturers recommend a 50:50 concentrate. I'm not so sure about the sales person's theory about wind chill factors though!
There is a fairly thorough description on this website outlining the properties of cooling system additives.