State Highway 25 at the north end of Thames will remain closed until an old mine shaft beneath the road is covered over.
Part of the road sank last week and the Transport Agency says a smaller hole appeared at the same place on the highway in January.
Waikato state highway manager Kaye Clark says excavations earlier this year indicated the shaft had been backfilled.[image:5076:half:right]
She says more extensive excavation following the latest sinking has now exposed the full extent of an old pump shaft.
Mrs Clark says the shaft, which is about six by four metres, will have to be bridged over.
A detour is in place but the agency does not expect motorists to be significantly delayed.