Deep underground gold mining was carried out in the Martha Mine, Waihi between 1892 and 1952 (McAra 1988). The mining technique was predominantly shrinkage stoping (excavation within an ore deposit or lode). Some of the stopes were 500m below ground level. In some areas after 1908 the deep abandoned stopes were not backfilled after the ore had been removed and there is reason to suggest that recent sink holes which have appeared in Waihi were initiated by collapse of cover material into the empty stopes. There have been two large sink holes, one in 1962 and the most recent one which occurred on the 4 February 1999. It was 40m in diameter and 20m (visible) deep. It appeared rapidly and apparently without warning just to the north (within 50m) of Seddon Street – one of the main arterial streets in Waihi. Today, mining at Martha Mine is by open cast, and old stopes are dug out as the mine goes deeper. However, some of the old stopes extend beyond the outer boundary of the open cast mine and lie directly under parts of the town. The risk of further sink hole collapse is very real to the people of Waihi and to the local council (Hauraki District Council, HDC).
Two sink hole collapses have occurred since mining ceased in 1952. Both occurred above the Royal Lode. One was in 1962 (PSM 1999) and the other on the 4 February 1999, Figure 1. The sink holes were very large (40m diameter, 20m deep) and appeared approximately 50m to the north of Seddon Street. Fortunately, no one was affected, but this can only be attributed to good luck. The February 1999 event occurred in public park land during the early hours of the morning.
