Under a Currency Interchangeability Agreement in 1967, the Brunei dollar is interchangeable with the Singapore dollar at par. As such, the Brunei dollar is accepted in Singapore as "customary tender"; likewise, the Singapore dollar is accepted for payments in Brunei.
Coins were used in Brunei from the 10th century. The Straits dollar was also used in Brunei from 1906.
History of coins used in Brunei
Due to the close ties between China and Brunei, the first type of coins used in Brunei were Chinese coins. This was initially called ‘Pitis’. They were later known as ‘Kue’ when local ‘Pitis’ were introduced. The local ‘Pitis’ coins had ‘Sultanate of Brunei’ stamped in front of the coin and the royal umbrella was imprinted at the back. These were issued from the 16th to the 19th century. Previous Islamic coins were also called the ‘Pitis’. Another type of coin that was used in Brunei were ‘Duit besi’ (which roughly translates to ‘Iron money’). Iron was considered valuable those days that it was used as money. 100 one-square inch pieces were valued at 1 dollar.
The last coin to be issued before the introduction of the Straits Settlements currency was the ‘Duit Bintang’, otherwise known as the ‘Star coin’ or the 'Star Cent'. It is called the Star coin because of the star imprinted on the obverse of the coin. It was minted in Birmingham, England, in 1887. It was made from copper.
With the introduction of the Straits Settlements currency, the previously used coins were taken out of circulation. They were, however still used with certain exchange rates.
History of banknotes used in Brunei
One Straits dollar banknote from 1935
The Straits dollar was introduced in Brunei in 1906. It was later replaced by the Malayan dollar which was introduced to British colonies and Brunei in 1939. It replaced the Straits dollar at par with a 1:1 exchange rate. The Malayan dollar was issued by the Board of Commissioners of Currency in Malaya. The board stopped issuing the Malayan dollar during the Japanese invasion during World War II. The Malayan dollar had the portrait of King George VI in front of the note.
In 1967, coins were introduced in denominations of 1, 5, 10, 20 and 50 cents. Except for the bronze 1 cent, the coins were struck in cupro-nickel. In 1986, copper-clad steel replaced bronze. Later, in 2008, the 1 cent coins switched compositions to brass.
On 12 June 1967, the government (Kerajaan Brunei) introduced notes in denominations of 1, 5, 10, 50 and 100 dollars. Notes for 500 and 1000 dollars followed in 1979. In 1989, the title on the paper money was changed to Negara Brunei Darussalam, the official name of the country, and the Malay term for “State of Brunei, Abode of Peace.” 10,000 dollar notes were introduced the same year. All notes bear the denomination in Malay (in both Rumi and Jawi) and in English. The English denomination appeared on the obverse below the denomination in Malay on the earlier series, but now appears on the reverse together with the Jawi.
Five series of notes have been issued. The colours of $1, $5, and $10 notes have been the same for all the series of banknotes. 
Second series – This series was the same as the first series with exception that the portrait of Sultan Omar Ali Saifuddin was replaced by the portrait of Sultan Hassanal Bolkiah, the 29th and current ruler of Brunei. All subsequent currency has the portrait of Hassanal Bolkiah. In addition, two new higher denominations were issued in 1979.
$1 ~ $100 like 1967 series
$500 – pink
$1000 – yellow
Third series – the post independence series. This series was gradually being replaced by the fourth series.
$1 – blue
$5 – green
$10 – red
$50 – brown, green, orange
$100 – purple
$500 – orange
$1,000 – red-violet, purple, olive
$10,000 – green, orange
1996–2000 polymer and paper series
Fourth Series (1996–2000) all notes except for the polymer issues are no longer printed.
Polymer banknotes were introduced in (2004) due to high cases of banknote forgery. All of them are polymer. The $100 note of this series has won a gold medal award for its security features in the 22nd National Print Award in Australia in May 2005.
The S$10,000 and B$10,000 notes are the world's most valuable banknotes, worth USD 8000 as of September 2014 (that are officially in circulation). They are worth eight times as much as the next most valuable, the 1000 Swiss franc note (USD 1063).
To commemorate the 65th birthday of Sultan Hassanal Bolkiah. Shortly after the notes were issued, the Braille dots on the upper left front corner of the new polymer notes are not raised. The Braille dots cannot be felt tactilely, and they are not accurately rendered as Braille numbers corresponding to the denominations. Specifically, the spacing of the dots is wrong, and they lack the lead-in character that indicates that numbers follow.
$25 – purple and beige (1992)
This was issued during the silver jubilee (25th anniversary) of Sultan Hassanal Bolkiah acceding to the throne. The design is of the 1989 series of currency.
$20 – yellow (polymer, 2007)
On 27 June 2007, Singapore and Brunei celebrated the 40th anniversary of the Currency Interchangeability Agreement (since 12 June 1967) by joint-issuing commemorative $20 notes.
The two authorities issue distinct versions of the new $20 notes. They are both yellow, 149 × 72 mm in size, and made of polymer. The reverses are almost identical except that the Brunei version has their state title in Jawi script, while the Singaporean version has the state title of Brunei in Latin script. The obverse of the Singaporean version is similar to the current Portrait Series, whereas the obverse of the Brunei version is similar to the $50 and $100 of the 2004 series.
There is a limited edition set, which consists of both versions in a folder, with matching serial number. The notes have "40th Anniversary Currency Interchangeability Agreement" overprinted on obverse. In addition, the Singaporean version has the two countries' state crests above the commemorative text. Only 12,000 sets are available, 10,000 from the Monetary Authority of Singapore, and 2,000 from the Brunei Currency and Monetary Board.
The circulation version has been available since 16 July 2007.
$50 - yellow (polymer, 2017)
In 2017, both Brunei and Singapore issued $50 polymer banknotes in commemoration of the 50th Anniversary of its Currency Interchangeability Agreement.
$50 - yellow (polymer, 2017)
In 2017, the Monetary Authority of Brunei Darussalam (Autoriti Monetari Brunei Darussalam) issued a $50 polymer banknote to commemorate the 50th anniversary of Sultan Hassan al-Bolkiah's accession to the throne.