They described the ward - Te Whare O Matairangi, Wellington's acute in-patient mental health facility - as severely overcrowded, under-resourced and unsafe - both for workers and patients.
The letter gave examples such as a homicidal patient showing up with a bag of weapons, and a staff member being sexually assaulted.
At the time it was written, most of the patients in the unit had a criminal or violent past including assaulting staff, significant stalking behaviour, arson and drug use.
PSA organiser Alexandra Ward said the 29-bed unit was over capacity for 11 months of the past year, sometimes caring for 34 people at a time.
"The mix of not enough staff and too many clients to be caring for with that understaffing.
"It is really consistent, it's really ongoing.
"The members have also been calling attention to it for literally for years."
The staff said they felt fearful and burned out.
Despite a recommendation from the Chief Ombudsman in 2017 to stop using seclusion and de-escalation rooms as bedrooms, the practice was still continuing.
The letter said at times agitated patients have had to be put in a courtyard because there was no other space for them and communal areas were also being used as bedrooms.
"The interview room and TV room are being utilised as bedrooms with mattresses on the floors.
"This results in increased frustration from the patients who are unable to access the television, or have private areas for review with their medical team," the letter read.
'Burnout and fear'
In another part of the ward, a female staff member was also having to support one sole female patient with sexually inappropriate men - which was impacting the ability to accurately assess patients.
The letter said staffing was at a crisis point with people leaving or taking sick leave.
"Lack of staff has also put more pressure on those working to continue on to do a 'double shift'.
"Pressure to provide safe staffing levels for work colleagues has been evident and many staff are feeling obligated to do extra work.
"The reality is that burnout and fear are both significant issues in many recent staff departures."
Ward said there was no let-up.
"It feels relentless and I think that's a feel for the stress and the burnout.
"It feels like every single week, members tell me that somebody else is leaving the service.
"We're talking about really dedicated, really passionate mental health support workers and mental health nurses who are just finding it easier to leave than to continue fighting for their clients and for their professional safety," Ward said.
The letter said the staffing gaps would be filled by new inexperienced graduates.
"This potential avalanche of raw recruits doesn't offer much solace to the few highly experienced registered nurses who remain."
Staff said security guards had been used to help make up numbers and while they were thankful for the support, their lack of mental health training and contractual restrictions was limiting.
In a statement, the Capital and Coast District Health Board said it was trying to fix the problems including hiring more staff and security guards, as well as training.
DHB Mental Health, Addictions & Intellectual Disability service general manager Nigel Fairley said despite Covid-19 delays the service had made significant progress over the past six months to address areas of concern.
He said it had done a review on the seclusion rooms and ordered furnishings that met requirements.
The service was also working to ensure safe staffing levels were being implemented.
