The move has sparked global condemnation.
Brunei on Wednesday introduced harsh new sharia laws, including death by stoning for adultery and gay sex, despite a storm of global criticism from politicians, celebrities and rights groups.
The previous law called for a maximum punishment of ten years in prison for gay sex.
Sexual contact between two women ("musahaqah") is not punishable by death in Brunei's Syariah Penal Code, but it is subject to brutal punishment involving two of the following three penalties: 40 lashes, a fine of up to 40,000 Brunei dollars (around USA $30,000), and a prison sentence of up to 10 years.
The Sultan of Brunei, Hassanal Bolkiah, has defended the law despite an worldwide backlash, defending Brunei's right as a "sovereign Islamic and fully independent country" to "enforce its own rule of laws...like all other independent countries".
He went on to call Brunei a "fair and happy" country.
Brunei's ruling royals possess a huge private fortune and its largely ethnic-Malay population enjoy generous state handouts and pay no taxes.
Sultan Hassanal Bolkiah instituted the code in 2014 to bolster the influence of Islam in the oil-rich monarchy of around 430,000 people, two-thirds of whom are Muslim.
The threat of the death penalty is apparently chiefly aimed at gay men, although experts are doubtful that executions will actually be carried out. Brunei Darussalam has always been practising a dual legal system, one that is based on the Syariah Law and the other on Common Law. The sultan had said then that the new penal code would come into full force over several years.
The first section was introduced in 2014 and included less stringent penalties, such as fines or jail terms for offences including indecent behaviour or skipping Friday prayers.
The final phase, enacted this week, makes sodomy between two men or unmarried heterosexual couples punishable by stoning to death or whipping with 100 strokes, Australia's ABC News reported.
But on Saturday, the government released a statement on its website saying the Sharia penal code would be fully implemented on Wednesday.
Phil Robertson, the deputy Asia director for Human Rights Watch, called on the sultan to "immediately suspend amputations, stoning, and all other rights-abusing provisions and punishments".
He acknowledged that a boycott is unlikely to change Brunei's laws, but said consumers must decide whether they want their money to support laws that violate human rights.
" Brunei's decision to implement Phases Two and Three of the Sharia Penal Code and associated penalties runs counter to its worldwide human rights obligations, including with respect to torture or other cruel, inhuman or degrading treatment or punishment", State Department spokesman Robert Palladino said in a statement.