Athlai
Not Terrible
New Zealand is a lot tougher for top 3 batsmen than middle order players and Williamson batting 3 is a plus point for him. I suspect South Africa, England and the West Indies are tougher for the top 3 though. NZ isn't uniquely difficult for no3s and as has been mentioned, the help for bowlers over there fall apart pretty quickly on a regular basis.
Batting records | Test matches | Cricinfo Statsguru | ESPNcricinfo.com
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The figures above show that NZ has the highest batting average for #3s of all Test countries since Dec 2012. However if you remove Williamson's figures from that NZ sample (71 innings averaging 71) the average for NZ is 33. His utter dominance is skewing perspectives here.
If you remove Williamson from the top 3 figures, the average of all other top 3 batsman in NZ since Dec 2012 is 32.45.
No other top 3 batsman comes close in NZ/SA/Eng/West Indies conditions.
Batting records | Test matches | Cricinfo Statsguru | ESPNcricinfo.com
stats.espncricinfo.com