Brunei's geography: a forested coast cut in two by Sarawak

Why Brunei is split into two pieces

Brunei lies on the north coast of Borneo, with the South China Sea to the north and the Malaysian state of Sarawak on every landward side. Britannica describes Sarawak as dividing the state into two disconnected segments of unequal size. The divider is the Limbang valley, which Sarawak annexed in 1890; the eastern piece left beyond it is the district of Temburong, and the western piece holds the capital, the oilfields and almost everyone else. The history article covers how the line came to be drawn there.

The total area on record is 5,765 square kilometres, of which Wikipedia's geography article, using 2009 figures, counts 5,265 as land and 500 as water. The other measurements of the edges disagree with each other. Wikipedia gives 161 kilometres of coastline, while the Convention on Biological Diversity's profile describes a coastline 130 kilometres long. Wikipedia's country article puts the land border with Malaysia at 381 kilometres, and its geography article at 266 kilometres. Neither article gives a date or a method for its figure, and both are printed here as found.

Until 2020 a driver from the capital to Temburong had to cross Sarawak through the town of Limbang, or take a boat across Brunei Bay. A road of about 30 kilometres linking the Muara and Temburong districts, much of it on a bridge across the bay, opened on 17 March 2020, and Wikipedia's article on the capital names the crossing as the Sultan Haji Omar Ali Saifuddien Bridge.

At sea, Brunei claims a territorial sea of 12 nautical miles and an exclusive economic zone of 200 nautical miles or to the median line. It is one of several states that claim part of the Spratly Islands, and it has disputed Sarawak's hold on Limbang since 1890; Wikipedia's country article reports that the Brunei government denies a settlement of 2009 that other accounts describe. This catalogue records both disputes and takes no position on them.

How the land rises from coast to Bukit Pagon

Britannica describes the country as a narrow coastal plain in the north giving way to rugged hills in the south. The two pieces differ sharply. In the west, the Convention on Biological Diversity's profile describes an alluvial and often swampy coastal plain backed by low hills, with more swamp inland; most of this interior lies below 90 metres and rises to almost 400 metres only in the far west. The east begins as a swampy coastal plain too, then climbs through low hills to mountains along the border with Sarawak.

The highest point is Bukit Pagon, at 1,850 metres, in the southeast on that border range. The lowest is the sea. Between them the land drains north: the Belait, Tutong and Brunei rivers cross the western piece, and the Pandaruan and Temburong rivers the eastern one, all flowing to the South China Sea. Britannica names the Belait as the largest of them. The capital, Bandar Seri Begawan, stands on the Brunei River about 14 kilometres from its mouth on Brunei Bay.

The soils are poor. Britannica describes them as deeply weathered, highly leached and generally infertile, with richer alluvium only along the rivers and in parts of the coastal floodplain, and white quartz sands in some areas. The arithmetic of farming follows from that: Wikipedia's geography article, from 2012 figures, counts 0.76% of the land as arable and 1.14% under permanent crops.

The coast itself varies along its length. The profile describes sandy beaches backed by a complex zone of estuarine mangrove and mudflat, and Wikipedia's article on the capital describes the coastal plains around Bandar Seri Begawan as low and marshy, especially to the south, with hills reaching the sea between Tutong and the capital and a strip of thick coal seams running along the shore between the capital and Muara. Britannica notes that Brunei historically depended on its rivers and the sea for transport, and that the rivers remain the main way into the interior while the road network serves the coast.

The ground is quiet. Wikipedia's geography article notes that Brunei lies near the Pacific Ring of Fire but that earthquakes are rare, and that the country lies outside the typhoon belt.

What the monsoon does to the rainfall

The climate is equatorial and hot all year. Britannica gives daily temperatures that typically fall to about 23 °C and rise to about 32 °C. Two monsoons set the rhythm: the northeast monsoon blows from December to March, the southwest monsoon from May to September, and April, October and November are transitional months.

Rain is heavy everywhere and heavier inland. Britannica puts the annual average at about 2,900 millimetres on the coast and more than 3,800 millimetres farther inland, with more rain from October to January and less from March to August. The wettest day on record, in Wikipedia's article on the capital, was 9 July 2020, when 662.0 millimetres fell at the airport.

Height changes the climate more than distance does. Wikipedia's geography article, describing the four districts, gives ranges of about 25 to 37 °C for Belait, 22 to 32 °C for Tutong and 20 to 36 °C for Brunei-Muara and the capital, and a cooler 18 to 29 °C for Temburong, where the southern uplands count as humid subtropical. The same article treats the whole country as having one climate with local variations.

Water is plentiful. Wikipedia's geography article, using 2009 figures, gives total renewable water resources of 8.5 cubic kilometres and annual withdrawal of 0.09 cubic kilometres, 97% of it for domestic use. The one environmental problem the article lists under current issues comes from outside the border: seasonal smoke and haze from forest fires in Indonesia.

Why so much of Brunei is still forest

Britannica estimates that about three-fifths of the country is virgin tropical rainforest and another fifth secondary forest. The Convention on Biological Diversity's profile, a draft prepared by its secretariat, gives 81% forest, of which 59% is primary and 22% secondary forest and plantations. Wikipedia's geography article reports 72% of the land under forest in 2020, about 380,000 hectares, down from 413,000 hectares in 1990, and says the whole of the forest area was in public ownership in 2015. Of the 2020 total, it counts 374,740 hectares as naturally regenerating forest and 5,260 hectares as planted, with 69% of the natural forest classed as primary and about 5% of all forest inside protected areas. That last share sits oddly beside the profile's figure for forest reserves, discussed below, and neither source explains the gap.

The three figures measure different things in different years, and all three put most of the country under trees. The profile gives the reason: most people live along the coast, and development has centred on hydrocarbons. Logging is limited by law. Britannica records legislation passed in the late twentieth century to restrict it, with plantations meant to supply only the local market, and states that timber is not exported. The profile adds that clear felling is prohibited.

The forest is not one kind. Britannica describes the undisturbed rainforest as made up mainly of dipterocarp hardwoods, especially of the genus Shorea. Freshwater and peat swamps cover the poorly drained lowlands of the Belait and Tutong rivers, mangroves line the lower rivers and the sheltered coast, and the profile lists the coastal kerangas forests among the sites that need protection. It describes the peat swamps of the Belait basin as still in almost pristine condition, and the mangroves of Brunei Bay as largely intact.

Which animals live in the swamps and forests

The profile estimates about 15,000 species of vascular plants, about 2,000 of them trees. Britannica lists proboscis monkeys, leaf monkeys, pigtail macaques, gibbons, sun bears, sambar deer and pangolins among the mammals, argus pheasants and hornbills among the birds, and the reticulated python among the reptiles. Two crocodiles occur, the estuarine crocodile and the false gharial.

The estuarine crocodile is the one that reaches the towns. The British Foreign Office's travel advice reports that saltwater crocodiles have attacked and killed people fishing on riverbanks around the capital and have been seen on beaches and in the ponds of the golf course at Panaga.

Offshore, the profile counts 45 square kilometres of coral reef with more than 50% live coral cover and about 400 species of reef-building coral. The Fisheries Department has placed artificial reefs in Brunei Bay, partly to deter illegal trawling.

Where most Bruneians live

The settled country is a coastal strip. Britannica estimates that about two-thirds of the population lives in and around Bandar Seri Begawan, and names the oil towns of Kuala Belait and Seria, on the southwestern coast, as the next most densely settled area. The forested uplands of the interior are thinly populated, mostly by indigenous communities that Britannica describes as practising shifting agriculture. The groups have their own ground. In Britannica's account the Iban and the Belait live mainly in the far west near the Sarawak border, the Kedayan in the central and eastern parts of the western piece and the Kadazan in its rural east-central area, while the Murut and the Bisaya have settled mainly in Temburong. Malays live in the river and coastal villages throughout the country, and the Chinese are concentrated in the towns.

The Department of Economic Planning and Statistics' estimate for 2025, set out in the table above, shows the same pattern by district: Brunei-Muara, the smallest of the western districts, held 332,700 of 458,600 people, and Temburong, the whole eastern piece, held 9,500. The department put the national growth rate at 0.7% in 2025, down from 1.1% in 2024.

Water shapes the capital as well. Kampong Ayer, the stilt village on the Brunei River, stretches about 8 kilometres along the river, and Wikipedia's article on the city gives it about 30,000 residents. Britannica describes it as the older part of the city, built on stilts along inlets of the river.

What protects Ulu Temburong and Tasek Merimbun

All of the country's notified protected areas are forest reserves. The Convention on Biological Diversity's profile states that forest reserves cover 41% of the land and that a further 15% had been proposed for gazettement, without dating either figure. It names six critical sites: the inland forests of Ulu Temburong, the peat swamps of the Belait river system, the mangroves of Brunei Bay, the swamps of Tasek Merimbun, the Bukit Batu and Sungai Ingei area next to Gunung Mulu National Park in Sarawak, and the coastal kerangas forests.

Ulu Temburong National Park covers 50,000 hectares of the eastern piece, according to Brunei Tourism, and spans lowland and mountain forest. Tasek Merimbun, in Tutong, is a blackwater lake inside a heritage park of 2,500 hectares, listed as an ASEAN Heritage Park. Brunei also takes part in the Heart of Borneo Initiative with Malaysia and Indonesia, which the profile describes as an effort to keep the island's forests connected across the three borders. Brunei became a party to the Convention on Biological Diversity by accession in 2008. The parks as places to visit are covered in Brunei: travel.