The reason that all or almost all of the power you use is renewable. Adding more renewable generation in the South Island won't help the coal generation in the North Island.
Right now, investment in infrastructure needs to be made to move power from Manapōuri to the North Island.
As to why we are not replacing the 1.8m tons of coal we import from as far away as Indonesia with wind or solar in the North Island? I don't know.
Edit: If you take a look here, as of an hour ago we are generating 90+% renewable, but with 192mw of coal generation. Wind is generating at a fraction of capacity and this probably accounts for the coal.
There is hydro capacity but that might be from dams far south.
It looks like there's just one HVDC interaliand link capable of sending 1.2GW, and dating back to 1964.
Why not install 7 more? That would allow the entire current demand for the entire of NZ to come from the south island.
10M USD per km, average 800km from centre of south island to Auckland, $8b in total. 43,000 GWh generation per years, that's just 2.5c per kWh over 10 years on your bill.
The current HVDC link has had a lot of upgrades from the original 600MW to 1200MW now, and there are proposals to upgrade it further, but not nearly to the scale you suggest, as there just isn't the need for such levels of transmission.
Unless there were plans for major new hydro schemes in the South Island there's no particular reason not to just build new generation in the North. There is ample wind and geothermal in the North Island.
This would all change if the Tiwai Pt aluminium smelter were to close as that would leave a huge amount of generation that would need to go north to be used.
Yes NZ is baffling. NZ is a place which could easily be 100% renewable with very little investment. There is plenty of wind, sun and the large hydro power station could serve as backup/storage. However I see dramatically less windfarms and solar installations than in a place like Germany which has much less sun and wind.
Right now, investment in infrastructure needs to be made to move power from Manapōuri to the North Island.
As to why we are not replacing the 1.8m tons of coal we import from as far away as Indonesia with wind or solar in the North Island? I don't know.
Edit: If you take a look here, as of an hour ago we are generating 90+% renewable, but with 192mw of coal generation. Wind is generating at a fraction of capacity and this probably accounts for the coal.
There is hydro capacity but that might be from dams far south.
https://www.transpower.co.nz/system-operator/live-system-and...