They brought about a great scientific improvement. At NIWA's site you can now see detailed accounting of the methodology behind the adjustments:

http://www.niwa.co.nz/our-science/climate/information-and-re...

That was put together in December 2010. Before that, they'd been handled in an ad-hoc manner. Some of the adjustments detailed in the literature, some no doubt scrawled on a piece of paper on some professor's desk or sitting in some fortran code on a 5 1/2 floppy. The thing about climate science is that it is done by academics, and if you've ever been in academia you'll know what I mean... If they want us to turn the modern economy inside-out, it is fair enough to ask for a little more rigour.