It's pretty difficult to get an unbiased estimate of this isn't it ? I think your own personal experience is the best way to gauge the truth of the matter. So are you taking longer shots yourself ?
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The first factor is that deer numbers (and tahr at times) increased a lot in the South Island from about year 2000 onwards. Shots can be longer there, so a simple shift from animals shot in South vs North would increase the average range. More people shooting on farmland could have a similar effect.
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The second factor is equipment.
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This is partly due to new rifles being better than the .303 Brit (specially synthetic and laminated stocks), Reliable, Dialable scopes, more accurate factory ammo. When I personally changed from a MkIII* target barreled 303 to a Sako 85 7mm08, even using the same 2-7x32 VX1, I straight away started making several of my "longest shot ever" and my "tally" of unrecovered animals plummeted. Getting a Leupold RX-1000 and S&B PMII 3-12 and taking part in a number of Gillie's field target shoots (couple of thousand rounds down range) resulted in another small increment and I've shot half a dozen between 300-400m now. Who has shot a "know your limits" steel stage at 500m ? Now, I'm not saying NZ is a wealthy place but there are a lot more hunters who can afford this sort of gear now (and helicopter flights which were unheard of before 2000), so money does make a difference if you've got access to it.
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Another issue is marksmanship skills and, as others have hinted, my impression (from formal target competition) is that individuals usually get better with age. But we have a good new generation of hunters in their 20's and 30's who we couldn't expect to be highly skilled yet. There are even fewer prepared to put themselves to the test on paper than there used to be and community knowledge for sitting, kneeling, standing and unsupported in general is gradually fading.
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Photo by @Gillie
