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A Fire and Emergency NZ spokesman said two Delta Energy workers were safely evacuated from the tunnel.
Emergency crews in breathing apparatus entered the tunnel with thermal imaging equipment and gas detectors.
The crews discovered the source of the smoke was a faulty spotlight that had emitted a smoke haze.
The spotlight was made safe and left in the hands of Delta Energy and its staff, the spokesman said.
The workmen had followed the correct procedures, calling Fenz and exiting the tunnel, he said.
The Brewery Creek Tunnel was used for drainage and contained a number of pumps.