Except public transport isn't exactly great in this area, and not suitable if you are transporting things. Trains are slow and not all that frequent, and stations tend to be unpleasant places to wait. Wellington rail station is also not very central, which I believe is something they are working on with lightrail/trams. I suspect buses will struggle up TG due to the gradient.
I don;t seem a problem with cars, especially when many will be EVs in the next decade.
You don't build a billion dollar road with taxpayer dollars, and then not want those same taxpayers to use it.
The concern motivating the consideration of a toll is that bunch of people will abandon pubic transport in favour of the new road and this will increase congestion closer to and in the CBD. EVs are only a solution to pollution, they don't solve congestion, parking or risk to cyclists and pedestrians.