The difference between bedrock and boulder river beds doesn't really become apparent until you get into smaller steeper runs. Creeks are where the big differences can be seen. A bedrock creek will have more slides and falls while a bouldery creek will have more technical moves, slots, and the like. As you noted, the steep runs in NZ tend to be more bouldery so you can expect lots of tight and technical moves with consequences like sieves and undercuts. The style of whitewater is comparable to a lot of the classic eastern US steep creeks like the Green (above Gorilla) or the Upper Blackwater.
The big difference is that New Zealand, especially the west coast of the South Island, gets tons of rain and the landscape goes from sea level to alpine in less than 15 miles. That all means that the runs flood huge every year and the boulders are constantly moving. Crux rapids that scared people off runs in 2002 are now wide open boulder gardens while some nice pool/drop sections have been turned into disgusting sieve piles by landslides. The runs are always changing on the West Coast, so local beta is crucial. NZ is home to some of the most undercut and sievey whitewater I've seen (just as bad as anything on the Clarks Fork Yellowstone). That said, it's also really fun with lots of boofs and tight moves.
Be prepared for technical, pushy, and consequential whitewater if you plan to go creeking down there. Most of the classic South Island creeks are very committing because the only access is by helicopter, so hiking out is not really an option on most of the steep runs. And, be warned, kiwis are notorious sandbaggers, so if they say it's grade V you know it's the shit.
"A river is more than an amenity, it is a treasure."
-Oliver Wendell Holmes writing for the majority in New Jersey v. New York