I'd avoid Montieths or Macs, other than macs Wicked Blonde (pilsner), Sassy Red (ESB) or Copperhop (pale ale). The rest of their offerings are decidely mainstream, due to the fact that they are, well, mainstream. Macs is owned by New Zealand Breweries/Lion Nathan, DB owns Montieths. Nothing craft or micro about them anymore I'm afraid. The macs I mention above I think are brewed by Shed22 in Wellington, much more care, much more hops.
Speights too (again, Lion Nathan) is oh so mainstream, even their 'craft' range. I don't think any of them are particularly memorable.
So the good? Emersons. Anything Richard Emerson makes is exquisite. Try them all. You can see a list of stockists/outlets at
http://www.emersons.co.nz/
Theres a newish brewery in Invercargill that Richard Emerson himself rates highly, can't think of the name and I haven't tried so can't comment. Wanaka beerworks too would be a good stop.
Auckland, 2 fairly central must sees, and a couple more.
#1 Galbraiths alehouse. Real ale. Take a whole day.
#2 Cock & Bull, where you can find the wares of one of NZ's most awarded brewers Luke Nicholas. Take a whole day. Theres a couple dotted around, the one in Newmarket is pretty good. And just down the hill from Galbraiths.
#3 Shakespeare Tavern. A bit underrated I think, the beers are pretty good and its right in the central city.
#4 Hallertau, a fair way out of town but it'd be a great sightseeing trip.
http://www.hallertau.co.nz/