What we found out is that apparently this short tunnel is called the Okaihau or Kaikohe branch - it was a branch line railway that that joined the Auckland rail line of the National Rail Network of NZ and Otira.
It was the most northerly line in NZ and was intended to run all the way to Kaitaia - it opened in 1923 and closed in 1987. The work started in 1910, opened in 1914 and finished in 1923 after the end of the first world war.
The great depression in the 1930s proved to the downfall and the continuation of the line was abandoned in 1930. After the line closed the NZ Railways Corporation retained ownership of the trackbed in hope that forestry proposals would happen but in the 1990s most of the rails were removed. It is still owned the the crown.
More information here.
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A ShutterBug Explores
Groet Kees.