The sandfly is a fact of life in New Zealand. Forrest McCarthy called them the countries top predator, which is in a sense quite accurate. The “bloody sandflies” are both widespread and annoying, and coping with them requires a few special preparations.
Thankfully, sandflies are no where near as obnoxious as mosquitoes. Yankees familiar with early summer blackflies will find that their experience will transfer well, and the sandfly is merely a small variety of blackfly. Sandflies live around water, but dislike rainfall, as well as significant wind and intense sun. Humid, still, overcast days are their playground. They like to fly, silently, around humans until they find a bit of exposed skin for bloodsucking. Their bites cannot generally be felt until it is too late, if at all. These bites generally swell up in the typical fashion 12 or so hours after being bitten, and persist for a week or two. While the strength of individual reaction varies, I found them extremely itchy for 10 or more days after the fact. Salves provide temporary relief, and are especially useful right before bed.
Unlike mosquitoes, sandflies cannot bite through thin layers of clothing, so things like gossamer woven shirts are quite adequate to keep them at bay. Long sleeves and fingerless gloves are a good idea, as are long pants. These pants must be tucked into gaiters or socks, as sandflies delight in flying up to gnaw on your shins. Knee-high socks are a good back up in case your pants become untucked from your gaiters (while fly fishing this happened during almost every river crossing). Lastly, a scarf or buff which can be tucked into your shirt and pull up over the back of a hat to cover ears, neck, and chin will be welcome during heavy sandfly pressure.
DEET is effective against sandflies, until it is washed or sweated off.
As with most bugs, my favored strategy is to avoid sandflies as much as possible. The eastern drainages in the southern Alps are primarily rock, very windy, and under heavy orographic shadow. Pretty much my favorite kind of terrain anywhere, and not coincidentally a mostly bug-free zone. The sandflies in Fiordlands were pretty bad in camp, but they didn’t really follow us out on to the water while kayaking. They were present most places on the Heaphy Track, but cooler and generally windy weather kept them mostly at bay. The trailhead in Kohaihai was just enough off the beach to be sheltered, and the sandflies there were apocalyptic. Thankfully we didn’t have that anywhere else on the hike.
I got 80% of my sandfly bites while fly fishing, for several reasons. First, I had arrogantly dismissed the need for gloves. Second, the DEET washed off my hands constantly. Third, my pants came untucked in the river. Fourth, the fishing was so absorbing and difficult that I ignored the flies and had little idea how bad I was getting it. Lastly, fishing puts you rather inevitably in ideal sandfly habitat. With a few extra precautions, I would have suffered much less.
In conclusion, sandflies should be prepared for well, especially if you’re visiting the wetter and more verdant parts of New Zealand. Thankfully, they’re not as well armed as mosquitoes, and don’t produce a maddening buzz, so if you’re properly equipped they’re somewhat easy to ignore.