Government coal surveys and exploration by mining companies have established that New Zealand has an estimated 16 billion tonnes of in-ground coal. However, 80% of this is lignite in Otago and Southland. Of the balance, much of the easily won coal is gone. The remaining amount that could be recovered is relatively quite small and depends on geological, engineering, economic and environmental constraints.
North Island resources
North Island coalfields are located in Northland, Waikato and northern Taranaki. Most known areas of sub-bituminous coal in Northland have been worked out, and the last mine closed in 1955.
Thirteen coalfields, extending from Drury (30 kilometres south of Auckland) to Mangapēhi (20 kilometres south of Te Kūiti), are geologically grouped as Waikato coalfields. In these, there are 2 billion tonnes of in-ground coal. However, much of this is in seams that are too deep to mine. In 2020, opencast mines at Maramarua and Rotowaro were the only Waikato mines still working.
Waikato coals are all sub-bituminous, laid down in the Eocene and Oligocene periods. Seams in the north of the region generally have low to medium ash and low sulfur contents, while seams in the south have medium to high ash and sulfur contents. The main seam is typically 3–10 metres thick, but reaches 20 metres or more in parts of the Huntly and Waikare coalfields.
Coal measures is a name for coal-bearing sedimentary rocks, which include sandstone, mudstone and conglomerates as well as coal seams. Coal measures mined in New Zealand are generally a few tens to several hundreds of metres thick, and between 10 and 75 million years old.
There are five coalfields in northern Taranaki, of which the Mōkau coalfield near the west coast is the largest. Previous mining was on a small scale, and there has been no production since the last mine, in the Waitewhena coalfield, closed in 1990. In-ground resources amount to 380 million tonnes of sub-bituminous coal, but this is unlikely to be economically mineable.
The South Island’s West Coast region contains New Zealand’s only bituminous coals. Some have unusual properties that are in demand in international coal markets. There are 13 coalfields of various sizes between Greymouth and Seddonville (40 kilometres north of Westport). The Buller, Greymouth, Īnangahua and Reefton coalfields are the most economically important.
While there are large quantities of in-ground coal remaining in the region, most of the easily-won coal has been mined. Over three-quarters of resources are in the Greymouth coalfield (mostly underground) and Buller coalfield (mostly opencast), but how much is mineable depends on coal prices, land access and a resumption of underground mining. After an explosion at the Pike River mine in 2010 which killed 29 men, the government decided that there would be no further mining in the area, which has been added to the Paparoa National Park.
Coal from the top of a mountain
One of the unusual features of the West Coast coalfields is that bituminous coal is found at high altitudes. In the 1940s, geologist Harold Wellman recognised that this was due to a complex two-stage geological history. Maximum burial occurred about 35 million years ago, then the deeply buried basins were lifted, so that they are now preserved in the mountains – a process later named inversion tectonics.
There are two main types of coal measures on the West Coast: late Cretaceous to earliest Tertiary Paparoa coal measures, and Eocene Brunner coal measures. Both Paparoa and Brunner coalfields contain seams up to 20 metres thick. Paparoa coals are characterised by low ash and sulfur contents. Brunner coals have similar properties but with varying sulfur contents.
West Coast coals are almost all bituminous, with a small deposit of anthracite at Fox River. Their properties see them valued as blending coals on world markets.
Otago and Southland
The bulk of New Zealand’s coal resources are lignites in the south of the South Island. Over 7 billion tonnes of mineable lignite resources have been proven in Otago and Southland, enough to provide a significant proportion of New Zealand’s energy needs for a long period if needed.
Otago coalfields include very large lignite deposits in Central Otago, the Kaitangata coalfield south of Dunedin, and several smaller coalfields. Central Otago lignites are in seams up to 90 metres thick and typically have 40%–50% in-ground moisture and low to medium ash and low sulfur contents.
The eastern Southland lignite fields are comparable to other large deposits in the world and are, by far, New Zealand’s biggest fossil fuel energy resource. They form extensive, multiple seams up to 18 metres thick, and typically have 40%–65% in-ground moisture and low to medium ash and low sulfur contents.
The Ōhai coalfield in central Southland has seams up to 23 metres thick containing sub-bituminous and bituminous coals, generally with low ash and sulfur contents.