One option would be to get a pair of Motorola T5720 radios—little two-way "walkie-talkies" that sell for $70 a pair here in the U.S. (www.motorola.com). They're very useful gadgets, and operate on both the short-range FRS (Family Radio Service) and the longer-range GMRS (General Mobile Radio Service) frequencies. To use the GMRS frequencies here in the U.S. at least, you need a Federal Communications Commission license, but that isn't a big deal—it costs $80 for a five-year license for GMRS use. Now maybe your Kiwi bureaucrats are a little less officious than our dear FCC friends, but New Zealand still has laws governing general-use licenses for the radio airwaves, so check before you buy.
Along with the radios, buy a pair of TalkAbout Speaker/Microphones ($40 each). These plug into the radio and then clip to a lapel or pack strap near your mouth. You simply press a button to talk, and hear the reply even when hands-off. It works pretty well for situations when you're arranging a belay or even halfway up a pitch.
None of the above equipment will really weigh you down that much—maybe a pound each for the radio, batteries, and speaker/mic. It won't be a fool-proof setup, as radio communications are line-of-sight and you'll be shocked at how easily the signals are disrupted by a rock or a tree. But for most situations it should work well.
Read reviews of a slew of other spangly trail electronics in Outside's 2004 Buyer's Guide .