Adultery and gay sex in Brunei will be subject to death by stoning from next week, authorities said, under a strict Sharia law that has been on hold for four years amid heavy criticism.

Rights groups reacted in horror Wednesday to the latest hardline move from the resource-rich nation on Borneo, which practises a stricter brand of Islam than its neighbours Malaysia and Indonesia.

The tiny sultanate will implement the harsh new penal code - which also prescribes amputation of a hand and foot for theft - next Wednesday.

Homosexuality is already illegal in Brunei but it will now become a capital offence. The law only applies to Muslims.

The new penalty for theft is amputation of the right hand for a first offence, and the left foot for a second offence.

Amnesty International Wednesday urged Brunei to "immediately halt" implementing the new penalties.

"To legalise such cruel and inhuman penalties is appalling of itself," Brunei researcher Rachel Chhoa-Howard said in a statement.

"Some of the potential 'offences' should not even be deemed crimes at all, including consensual sex between adults of the same gender."

A notice on Brunei's Attorney General's Chambers dated December 29 last year said the provisions will take effect on April 3.

Brunei first announced the measures in 2013 but implementation has been delayed as officials worked out the practical details and in the teeth of opposition by rights groups.

Under a shift towards hardline Islamic law, Brunei in 2015 banned excessive Christmas celebrations for fear that Muslims could be led astray.

Brunei's Sultan is no stranger to controversy at home - the monarchy was deeply embarrassed by a family feud with his brother Jefri over the latter's alleged embezzlement of $15 billion during his tenure as finance minister in the 1990s.

Court battles and investigations revealed salacious details of Jefri's un-Islamic jetset lifestyle, including claims of a high-priced harem of foreign women and a luxury yacht he owned called "Tits".