I was a bit underwhelmed by the quality of the work a couple of young fellas from Peter Jackson Plumbing did when the landlord had to get them out to the house I was renting last year.

Not exactly a recommendation as I don't have any direct experience of their work, but I know one of the guys from Thomas Plumbing, local family run business and as far as I know they aren't cowboys.

I rate Graeme Sinclair plumbing (based in Lower Hutt).  Not the cheapest by any means but they have come to my rescue a couple of times at short notice and their customer service and plumbing/gas fitting skills are quality.  I haven't had them replace my hot-water cylinder but they have done gas fitting and plumbing work for me and my house has neither flooded nor blown-up.