What intterupted the building of the Homer tunnel?

The Homer Tunnel linking the Upper Hollyford with the Cleddau River was one of the projects of the 1930s depression, designed in part to get a lot of troublemakers out of the towns. The same was true of forestry in the Kiangaroa Desert (as it was known), the West Coast road south of Haast, and several similar.

Because Milford Sound is an almost perfect harbour, easily defended, it was necessary to deny its facilities to the enemy. So when WWII broke out, it was decided to at least continue the tunnel work to the point of 'break through'.

Another item that caused delay in work, was a giant avalanche which smashed flat the concrete tunnel entrance.

[IMHO the Hollyford entrance was very poorly sited, guaranteed to suffer from avalanche.]

Work recommenced in the late 1950s but even today, it is far from a perfect example of tunneling.