Four foreign domestic workers from Indonesia have been arrested in Hong Kong after they were caught offering dental services despite having no training, authorities revealed on Friday.

The workers, aged 31 to 35, rented a room in a guest house on their weekly rest days or holidays to provide dental services for their countrymen.

According to a preliminary investigation by the Immigration Department, none of them had received formal dental training and they were not registered dentists in the city.

Officials said their contractual employers were unaware of the unlicensed dental practice. The quartet were detained for breaching their conditions of stay.

“The arrested helpers have breached their conditions of stay,” a department spokesman said. “Apart from that, practising dentistry without registration will also seriously threaten the health and lives of Hong Kong citizens and will very likely increase the risk of the spread of the epidemic. The situation is absolutely unacceptable.”

Authorities seized business cards and an account book when they arrested the group at their respective contractual addresses on Thursday and Friday.

The department said the investigation was still under way, adding it would not rule out the possibility of more arrests.

The law states that a domestic helper should only perform duties specified in their contract at the employer’s household. They also should not take up any other employment.

Under the Immigration Ordinance, the penalty for breaching a condition of stay is a fine of HK$50,000 (US$6,435) and two years in jail. Under the Dentists Registration Ordinance, the penalty for pretending to be or taking or using the name or title of a dentist is a fine of HK$100,000 and three years behind bars.