This includes non-co-operation with clerical and admin duties
Around 4,500 psychiatric nurses around the country have begun industrial action over staffing and pay.
The Psychiatric Nurses Association (PNA) says its members are angry at a lack of recruitment.
It says there will be about 1,000 staff vacancies in the system by the end of the year.
They are engaging in a work-to-rule after 87% of PNA members voted for industrial action.
This will include non-co-operation with clerical and admin duties.
Peter Hughes is the PNA general-secretary designate.
He told Newstalk Breakfast this is just the first step in their action.
"This is ongoing and will be built on in stages," he said.
"We may look at the option of asking our members not to volunteer to do anymore overtime."
He says there are currently over 600 vacancies, which could rise to 1,000 within the next six months.