There are two potential issues with drinking water in the South Island or Rakiura.
Water purity and hand hygiene.
Much of this area has some of the cleanest water in the world, running in swift-flowing high country rivers and streams.
Rainfall is high and straight off the Tasman Sea. The streams get flushed out on a regular basis.
No large predators do what they do in overseas forests: bears, wolves, etc, in North America, highly populated Europe, or the murky, slow-flowing streams of Australia.
Herbivorous deer, goats, pigs, etc, are relatively few in number. Possums are more numerous but fairly small.
Any small amount of animal waste that finds its way into a stream is speedily flushed away and diluted.
Of course, you would avoid drinking slow-moving water when going through areas with numerous livestock or when the water has bright green slime.
Or from alpine tarns, where birds can contaminate the water purity.
You don’t need to filter the water from fast-flowing rivers or streams. Save your time and energy.
That’s not where water is a problem.
Major waterborne diseases, like Norovirus or Giardia, happen sporadically and are usually well-publicised.
At huts!
These diseases begin with poor toilet hygiene, and then people spread the illness by touching taps without washing their hands or other similar methods.
So hand hygiene around huts is where you should expend your effort. Around the long drops, watertank taps, and sinks.
Soap is what is required to kill those germs.
Maybe sometimes at campsites with no toilets, because people can be disgusting.
Once again, it is poor toiletry hygiene.
That means on the South Island or Rakiura, there is little benefit in carrying a water filter and attempting to purify remarkably pure water.
Do it in other countries?
Sure.
But it’s the wrong problem in New Zealand.
Therefore, you often don’t need to carry much water. You are regularly walking past streams unless you are on the ridges of the Alpine route. Maybe trucking over Waiau Pass, except you can drink at the base of the scree climb, and then there is pure water available in the valley on the other side.
It’s your own and other people’s hand hygiene you need to deal with, not the pristine water flowing in mountain streams and rivers.