The India side: the whole gain at 12.5%
India taxes the full capital gain from your original cost. For long-term property sold on or after 23 July 2024, the rate is 12.5% without indexation, and as an NRI you get neither the resident option of 20% with indexation nor any currency relief for the rupee's fall, so the whole rupee gain is taxed. The buyer must deduct TDS under Section 195 on the gain, plus surcharge and cess, not the small 1% that applies to a resident seller. A lower-deduction certificate reduces the withholding, and any excess is reclaimed by filing an Indian return.
New Zealand: usually nothing, unless the bright-line bites
New Zealand is unusual among the countries Indians move to in having no general capital gains tax. So a gain on selling your Indian property is, as a rule, simply not taxed in New Zealand, and India's 12.5% is the only tax you pay. For most people selling a property they have owned for years, that is the whole story.
The exception is the bright-line test. If you sell residential land within two years of buying it, that quick gain is taxed as income in New Zealand, and importantly, the test applies to overseas residential property held by a NZ resident, not just NZ land. Where it bites, New Zealand taxes the gain but gives a credit for the India tax, since India has the primary right to tax Indian property. A separate shelter can also apply: a recent migrant within the roughly four-year transitional-resident window is generally exempt in NZ on this foreign-sourced gain anyway. So the practical rule is: a long-held Indian property is taxed only by India at 12.5%, and only a sale within two years of buying brings a New Zealand charge, with a credit.