During the present century Auckland has been the most rapidly developing part of New Zealand. It has experienced consistent inwards migration, especially heavy between 1906 and 1911, slackening between 1926 and 1936, but accelerating since then. Up till 1930 progress depended mainly on the rapidly expanding dairy industry in the Waikato, Bay of Plenty, and Northland districts. Closer settlement stimulated growth in a large number of provincial towns, the most important change being the emergence of Hamilton by 1926 as the second largest urban centre. In the first decade of the century the acreage of sown pastures increased from less than 2 million to 3·3 million, and by 1931 it had reached 5·8 million. In 1920 the volume of dairy products exported from Auckland for the first time surpassed that from Taranaki. Government purchase of large estates played little part in the new wave of settlement, but by carrying out extensive regional drainage schemes on the Hauraki Plains and Rangitaiki Plains the State undertook its first major land-development schemes. The kauri-timber and kauri-gum industries were in steep decline by 1920 and the sawmilling industry had moved to the King Country and the interior volcanic plateau; gold mining survived only at Waihi, but the Waikato sub-bituminous coalfield produced a steadily bigger proportion of New Zealand's coal output–from 10 per cent in 1960
The first attempts at closer settlement of the pumice lands of the central North Island failed because livestock suffered from a wasting disease known as “bush sickness”. Large areas of scrub- and tussock-covered land were therefore regarded as agriculturally useless, but seemed admirably adapted to tree growth. The rapid depletion of native forests and the spectre of impending timber famine made both State and private interests favourable to large-scale forestry. Between 1925 and 1936 more than half a million acres of exotic conifers, mainly pinus radiata, were planted in the pumice country. The discovery of cobalt deficiency as the cause of bush sickness opened the way to renewed agricultural expansion in the 1930s and to an accelerated programme of land development by the State after the Second World War. Recently State land-development operations have also been carried out on “gumland” soils of Northland and in the hills of the northern King Country, where much land reverted and was abandoned during the depression of the 1930s.
Two other major developments since the Second World War have been the construction of a geothermal power station at Wairakei and eight hydroelectric-power stations on the Waikato River, and the establishment of large-scale timber and pulp and paper plants to process the rapidly maturing exotic forests on the Kaingaroa Plains and the upper Waikato valley. These developments, together with land settlement and the popularity of Lake Taupo, the Bay of Plenty coast, and Rotorua as recreational centres, have led to spectacular expansion of towns since 1945. Tokoroa (7,100 in 1961), Kawerau (4,500), and Murupara (1,570) were scarcely names on the map in 1945, while the Tauranga urban area has grown from 6,400 to 24,600 between 1945 and 1961 and the Rotorua urban area from about 9,100 to 25,000. Harbour improvements at Mount Maunganui have broken Auckland's long virtual monopoly of the external trade of the provincial district, and the Port of Tauranga in 1960 ranked fourth by volume (but not by value) of goods handled at New Zealand ports.