The Homer Tunnel was started construction in the 1930s depression as one of the projects used to withdraw single men [= troublemakers in some eyes] from the cities.
[Similar projects for similar reasons were the planting of the Kaingaroa Forest, and the road down the Hollyford, intended to reach Haast.]
When WWII started, it was considered that as Milford Sound would offer a perfect harbour to an enemy, the tunnel should at least be broken through; and that done, the work on the construction halted.
After the war, work re-started, but a huge avalanche crushed the concrete portal, and killed several workers, and work halted for some time. Even today, the tunnel entrance is covered a few times each year, or at least the danger is very high.
When first I visited in the early 50s, there was no public transport through the tunnel - one had to walk through and pick up a second bus on the far side. The Milford Track itself had yet to be re-opened after WWII. It was overgrown, and the tourist huts had been damaged.