Most people know that there was a major deviation of the North Island Main Trunk line between Mangaweka and Utiku that was opened in November 1981. This is covered on one of my maps of the North Island Main Trunk, as shown below.
The tunnel locations are as near as possible attempted to correspond with the NZ Railway and Tramway Atlas 4th Edition and are based on observable locations.
Part of the route is in a public walkway, this at this time does not appear to include any of the tunnels. The part which is accessible appears to be roughly between Tuna Road and Te Kapua Road.
Two video clips posted on Youtube (shown below) appear to be of Tunnel 10A, which by measurement from Google is approximately 1.2 km south of the Mangaweka Viaduct and is not on a publicly accessible route. A slip which is stated to have occurred at the north end of this tunnel is easily visible on Google.
Whilst most of the remaining tunnels can be seen clearly, there is reasonable grounds for speculation about the state of the railway formation around the northernmost tunnels due to the very visible land movement occurring in an area in which the soft papa/limestone soils are constantly on the move. This is exacerbated by the fact that this portion of the railway route passes along the bank of the Rangitikei River. The erosion and slipping of the banks of the river are observable along a substantial length of the river for many km north and south of Mangaweka. Fortunately the NIMT does not follow this river for a great distance. However the aerial coverage in Google suggests that some risk exists around the Makohine Viaduct area. The new route sidesteps the worst part of the riverbank although it still has to cross the Rangitikei River twice on two large viaducts.
View Larger Map
Here is the map. The old route shown in aqua to the left and the current route shown in red. You can see fairly clearly what I mean about the state of the river banks. Zoom in on the map and drag it to see some of the features described. Tunnel 10A is the first tunnel on the old route (starting from the bottom of the map and working your way north).
Here is the first of two videos from Youtube. This one shows the south end of the tunnel.
The second clip is below and shows a walkthrough of the tunnel.