The province's police watchdog is investigating after two Niagara police officers shot someone with an anti-riot weapon before arresting them under the Mental Health Act.

In a release issued Monday, Ontario's Special Investigations Unit (SIU) said it has assigned three investigators and one forensic investigator to look into what happened between the officers and a 46-year-old man in St. Catharines earlier that day.

The SIU, which investigates police interactions that involve a death, serious injury or sexual assault, say they believe the officers "discharged Anti-Riot Weapon ENfields," also known as ARWENs, at the man around noon. An ARWEN is a so-called "less lethal" launcher that can shoot items including plastic projectiles, chemical irritants or smoke canisters.

The agency said its preliminary information "suggests" police were called to help a man in distress near Dunkirk and Bunting roads in St. Catharines around noon Monday, found the man inside his vehicle and made attempts to speak with him.

"At some point, two officers discharged their ARWENs at the man," the release said. "He was apprehended under the Mental Health Act and transported to hospital."