State Highway 25, north of Thames, is closed as contractors work to cover a mine shaft which was exposed when the road sank last week.
The NZ Transport Agency says the shaft is the same location as a smaller hole which appeared in the road in January.
An old pump shaft was exposed when the road sank last week.
State Highway manager Kaye Clark says they excavated the holed in January and initial investigations indicated the shaft had been back filled.
“It seemed some settlement in the fill was what caused the road to sink.
“This time we have excavated much further and have now exposed the full extent of an old pump shaft.”
Kaye says historic records indicate this may be the site of the first ’Big Pump’, constructed for the Thames goldfield in 1872 and closed down in the early 1900’s.
The shaft is about 6m by 4m and it is unknown if it had been back filled previously or how it was covered over as there is at least five metres of water in the shaft.
Local historians have been involved and discussions are also being held with the Historic Places Trust to ensure the shaft is well documented and recorded.
“The hole will require bridging over to allow the road to be reinstated and design work on this is currently underway.
“It is essential we provide a long term solution and having to span such a wide opening so as not to put any loading on the shaft’s walls is providing our engineers with some challenges.”
Kaye says until this work is underway, it’s not possible to say how long the highway will remain closed.
She says the NZTA will be carrying out the work swiftly as possible to minimise disruption to traffic.
A short local detour is in place and is not expected to cause significant delays for motorists.