After the incident, Betsi Cadwaladr of the health board apologised and said that it was contacting the patients who were mistakenly discharged.
Earlier, the health board's interim chief executive Simon Dean told a Senedd committee that discharging the patients was an "error that should not have occurred".
According to Simon Dean, 200 to 300 people were affected and his team was "working through the precise number".
But recently, in a letter to Plaid Cymru, the political party's health spokesperson Rhun ap Iorwerth confirmed that 1,694 patients had been wrongly discharged.
"To now learn that 1,694 patients were discharged early, while they still needed support from mental health services, is deeply distressing,” Mr. ap Iorwerth said.
England and Wales have registered 40 percent more deaths in hospitals this year on average, according to the UK Office for National Statistics.
Overall, the UK has so far confirmed 265,231 COVID-19 cases and 37,048 coronavirus-related deaths, according to the World Health Organisation.