The Hunua Ranges, roughly 25 miles south-east of Auckland City, extend from Firth of Thames in the east to almost the Great South Road in the west. Together with the lower Waikato River they form a natural geographical boundary between North Auckland and South Auckland Land Districts. The Hunua Ranges are essentially a series of tilted, fault-bounded Mesozoic argillite blocks capped by remnants of Tertiary rocks consisting of sandstones, siltstones, limestones, and coal measures. Following Late Tertiary block faulting, the lower lying areas in the west and south were filled with thick alluvial sediments, and basaltic volcanics were erupted at points along the numerous fault planes. The topographic expression is subdued in the vicinity of Waikato River, but rises progressively northeastwards to a maximum elevation of 2,200 ft in “The Thousand Acre Clearing”, beyond which the ranges are terminated by the Firth of Thames.
The Hunua Ranges first came into scientific prominence in January 1859, when a combined expedition of geologists, zoologists, botanists, and surveyors made a brief excursion along the western flanks to Mangatawhiri River where they embarked in three canoes and descended to the Waikato. The geologists of the party, Ferdinand von Hochstetter and Julius von Haast, reported on a coal seam, which was later mined at Drury. Continued mining and prospecting proved, however, that reserves were limited and were sufficient to yield only 8,000 tons before mining was finally abandoned in 1935. In addition to coal, manganese, limestone, and pumicite have been worked on a small scale, and road metal and concrete aggregate are at present being quarried at selected localities.
Perhaps the most notable economic feature of the Hunuas, however, is their contribution to Auckland's water supply. The final stages of the vast construction programme that will result in a total catchment area of 28,000 acres, a reservoir storage of 15,000 million gallons, and a safe mean yield of 68 million gallons per day, is now in hand, and when completed will, in conjunction with the yield from the Waitakere Ranges, ensure adequate supplies of water for Auckland City and suburbs at least for the immediate future.
by Barry Clayton Waterhouse, New Zealand Geological Survey, Department of Scientific and Industrial Research, Otahuhu, Auckland.