The brother of a Canadian teacher who has been detained in Indonesia on allegations of child sexual assault says the news that charges will be laid is "devastating."

Neil Bantleman, who worked at the prestigious Jakarta International School, was arrested during a police investigation into the alleged sexual assault of three kindergarten students.

His brother, Guy Bantleman, said from Toronto that the prosecutor in the case has not released the details of the charges but is expected to do so later this week.

He adds that his family had hoped for Bantleman's quick release when he was detained along with teaching assistant Ferdinand Tjiong in July.

Both men have denied the allegations since their arrest.

Indonesian law stipulates that police can't hold someone for more than 120 days without pressing charges, and Guy Bantleman says his brother has been in prison for 108 days.