I don't think we've got any New Zealand collectors on here, it definitely looks like a relatively good one though. You'll need to check with local collectors to find out how valuable this specific bottle is, it all depends on rarity. Or if you want to sell it put it up as an auction on Ebay and see how much it goes for. I suspect it won't be super rare because Grey and Sons was probably on the larger side as far as New Zealand bottlers go. I actually found a bottle in a thrift store in Canada from their successor, Grey & Menzies, made decades after yours, so it was a long-established company.