As well as having the Cromwell Gorge to look at, I have now decided to take a look at Johnsonville-Tawa at the same time. The original 1885 route of the NIMT was built as the Wellington and Manawatu Railway and became the NIMT in 1908. Due to the steep gradients and consequent limitation of capacity, the Tawa Flat Deviation was constructed and opened in 1937. The route lay abandoned until the late 1940s when work began on the Johnsonville Motorway.
I have determined that Archives New Zealand have in their collection a set of contact prints taken in 1944 covering the footprint shown above. The camera used was a Williamson Eagle IV and the negatives are at a scale of 1:8000.
This is not a large section in length and with plenty of overlap I won't have to reorder additional images as I am having to do with the Cromwell Gorge section.
Depending on ANZ timeframes I would expect to have the scans in a week or two.