If you plan on shooting a lot then .308 as the ammo is cheap and it will knock over anything in NZ.
If you do really plan on going 500yd+ then sell the .308 and start looking at 300 win mag and 7mm rem mag etc, but remember that the ammo is pricey, they burn barrels and to make the most of them you will need a pricey scope. They're also pretty overkill for NZ at normal ranges.
Don't get tied up with "flat shooting", they're all similar out to 400 and past that you need to be dialing/compensating anyway.
That's just my opinion, you'll find plenty more on here.
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