An excellent construction underway to replace the old viaduct which was structurally inadequate when it was built. See the details and pictures here: Newmarket Viaduct
The old viaduct had to be strengthened after construction with post-tensioning cables installed. During my civil engineering training at Auckland University in the early 1970s, our structures professor explained how it was found to be inadequate and was strengthened.
Being the main southern artery out of Auckland city, the structural risk is too great. It would be an economic disaster if a minor earthquake rendered it unusable.
Some people mistakenly think the replacement is purely a capacity increase project. While one more lane is being added, that's not going to be of much use to improving motorway capacity because going both north and south, there's no capacity issue on the viaduct. The bottlenecks are on either side because of joining traffic at Saint Marks Road and Gillies Avenue as well as bottlenecks entering Auckland city and over the harbour bridge at peak times.
The precast cantilevered construction is easy compared with the construction of the old viaduct which involved huge amounts of concrete poured in situ with scaffolding galore.