Stumbled across a nice flyover video: https://www.youtube.com/watch?v=kwNrsfPiIkc
Road follows a river and railroad track through a national park. Realignment is not adding lanes, but smooths curves and improves sightlines. More info: https://www.nzta.govt.nz/projects/sh73-arthurs-pass-mingha-bluff-realignment/
Looks like a nice place, and a lengthy project. The old road should be closed off and restored, since it's in a national park. It would be expensive though.