A chemistry teacher at a Hong Kong secondary school has been arrested for allegedly putting instructions online on how to make a poisonous gas.

Police officers from the Cyber Security and Technology Crime Bureau spotted an article on an online discussion forum in mid-October that detailed how to make hydrogen sulphide, a highly toxic, flammable gas, Chief Inspector Cheng Chak-yan on Thursday said.

The instructions featured images and listed the raw materials needed, weights and mixing ratios, Cheng said, adding the amount of harm that could be inflicted with different concentrations was also provided.

Police have said the suspect is believed to have used his position as a chemistry teacher to obtain the necessary materials to make the poisonous gas.

The teacher also listed some applications of the toxic gas, which smells like rotten eggs, including the production of stink bombs.

An investigation led officers to the 30-year-old man, a teacher at a Tin Shui Wai secondary school. He was arrested in Tuen Mun on Thursday morning on suspicion of inciting others to administer a noxious substance with intent to injure.

Police said they suspected he used his position as a chemistry teacher to obtain the necessary materials from the school lab and said he uploaded photos outlining the steps to make the gas.

The force said the teacher also published an article online to incite others to make a device able to shoot out fire in July 2019, during the anti-government protests.

Two mobile phones, a laptop computer and 1.8kg of chemical compounds were seized during the operation on Wednesday.

Police said the 30-year-old had been detained for questioning and did not rule out the possibility of more arrests.