But then, in the distance, a glimmer of light appears and walking slowly becomes easier.
The light comes from a hole cut in the rock wall of the tunnel, which looks out on to the rugged Waitawheta Gorge and the Waitewheta River, which powers its way through the rocks about 35m below.
This is the Windows Walk - named for the four openings like this which provide intermittent illumination along the tunnel - which forms part of the wonderful network of walking tracks focused around the Karangahake Gorge between Paeroa and Waihi.
Last time I explored the gorge, about five years ago, DoC was busy upgrading the Windows Walk so I wasn't able to check it out.
Instead, I gazed hopefully up at the mysterious rock windows from the Crown Tramway Track on the other side of the river - half expecting to see a couple of bearded dwarves peering grumpily back - and made a mental note to come back when the upgrade was completed.
That has long since happened and DoC has extended the original tunnel by 70m, to form a link with the network of tracks up the Waitawheta Gorge, restored the tramway rails and put gates across the various mine shafts which aren't safe to enter.
I planned to check out the new route as part of a big loop around the track circuit, which I guesstimated would take four to five hours.
Unfortunately, when this scheme finally came to pass, my journey there started under grey skies: I ran into drizzle around Kerepehi, hit rain at Paeroa and reached the Karangahake Gorge in a torrential downpour.
Sadly, in the face of all this wetness I chickened out and decided to do a much smaller loop, taking advantage of the shelter provided by the tunnels.
This short walk involved parking at Karangahake Hall, where locals were setting up the Saturday market, bouncing across the swing bridge over the Ohinemuri River and walking through the fantastic old 1km railway tunnel, home to lots of glow worms and, these days, some useful electric lighting, to join the Karangahake Historic Walkway which leads to the Crown Track.
At the start of this track, where the Ohinemuri and Waitawheta Rivers meet, are the impressive ruins of the Talisman and Woodstock Batteries, built more than a century ago to crush rocks extracted from the Woodstock mine.
The tunnels which today form the Windows Walk were excavated as part of this mine, which finally shut in 1918.
The walkway was actually the route of the old horse-drawn tramway used to carry quartz from the shafts to the two batteries, where it was crushed by 90 giant stampers so the gold could be extracted.
The tunnel windows were cut so any spoil excavated by the miners could be dumped directly into the river below. After I'd shuffled my way through the tunnel, a stairway led me to a bridge across the river to the remains of the vast underground Woodstock pump house, built to power this huge operation.
I tried exploring the pumphouse tunnels using the technique that got me through the Windows Walk but after about a quarter of an hour I conceded that DoC was right to advise the use of a torch and retreated to the daylight.
From there I had hoped to walk back to the car via the Crown Tramway Track and the ruins of the Crown Battery but that route was blocked by a massive slip.
But there are walks and bridges everywhere here so it was easy enough to stroll back down the other side of the river instead.
It was still raining heavily but thanks to all the tunnels, the overhanging cliffs and the thick bush growing on either side of the gorge I was still surprisingly dry.
The photos used on the display boards along the walk show scenes of utter devastation from when gold mining was at its height - very different from what's here today.
The tracks and even the giant ruins, with their towering concrete walls and chunks of rusting iron, are now clad in green and the rivers, once choked with discarded dirt and rock, are now clear and sparkling.
So clear, in fact, that back at the final bridge there was a man swimming in the river. Was he, I wondered, searching for gold?
After a few hours walking in the rain I, on the other hand, was searching for hot coffee, which was readily available at the nearby Talisman Cafe.
But when I returned across the bridge, warmed and refreshed, the swimmer was still there, seemingly still looking. Did he know, I wondered, that most of the gold was dug out of the gorge a long time ago?
CHECKLIST
Further information: You can download a map of the Karangahake Gorge tracks from the Department of Conservation's website.
Details of places to stay and tourist services in the gorge are at karangahakegorge.co.nz.
