Solution does not lay in adding more lanes on the Southern Motorway
Two very salient points below:
Because adding more lanes to the Southern Motorway north of Manukau or widening Mill Road to a 4-lane defacto expressway with a high speed roundabout outside a school is not the way to go.
So does your solutions look a little something like this?

Source: Auckland Transport and NZ Government

Simple Amsterdam Streets, Paris/German housing & Hong Kong PT.