Miners over a century ago tunnelled as deep as 600m.
"In some way part of the problem is the records aren't nearly as comprehensive as they should be, so it's difficult for us to say whether it's going to be any good," he said.
If samples showed commercially viable signs of gold and silver the life of the pit could be extended beyond 2014 when mining is due to end.
"If the Martha Exploration Pit indicates the potential for underground mining we would be required to commit to a full resource management consenting process," the company said.
The tunnel will be bored into the pit wall and be about 5m high by 5m wide and zig-zag down about 2.6km long.
It will end about 20m below the bottom of the pit. Work would take two to three years to finish and coincide with the completion of more work on the east wall of the mine.
The company said the project was "small" with less waste rock and ore moved during its two year life than about one month of open pit mining.
The project will not tunnel under any residential property.