All human efforts have some eco footprint. Hydro dams have similar CO2 emission profiles to wind and solar, so for argument's sake, let's assume their ecological impact is lesser than that of coal or natural gas. The south island of New Zealand is almost exclusively hydro, the north island has just a little bit of coal and natgas (less than 1GW of generation in aggregate) to displace to go 100% zero carbon for electrical generation.

There is an interconnect (Cook Strait cable, 1200MW HVDC) between the islands, and the south island ships power to the north island (primarily hydro), so you can aggregate renewable generation between both islands fairly effectively and treat them together as a single system.

Electricitymap.org South island: https://www.electricitymap.org/?wind=false&solar=false&page=...

Electricitymap.org North island: https://www.electricitymap.org/?wind=false&solar=false&page=...