4 lanes per direction? That does seem like overkill. Howabout expanding the road between christchurch and dunedin to 2 lanes (per direction), especially between ashburton and christchurch which is 1 lane (per direction) for the vast majority of it, and people commute it for work.

1 lane means no safe overtaking, and some parts don't have overtaking lanes for tens of kilometres. Milk trucks are the worst for driving slowly, though campervans are also pretty annoying.

Having high speed rail between the towns would be ideal, but christchurch's public transport is terrible, and proper rail would require a complete rework of all (or virtually all) the rail in the country. Busses might help reduce the rush hour load, but since they're limited to 90km (I think), it wouldn't work without also expanding the moterway.