"We have been asking for a new system for two years. We're still waiting."
The letter finishes by accusing Health Minister Jonathan Coleman of not listening to their concerns, and asking for improved funding.
It comes in response to a highly critical report into Wellington's public mental health services.
Five patients were involved in attacks over 15 months.
The Mellsop review found major problems in each case. It recommended a raft of changes to the system including better record keeping and faster assessments.
PSA national secretary Glenn Barclay said the mental health workers knew it was a big step to speak out publicly, but they felt they had no other choice.
"Under National more than $1.7 billion has vanished from the health budget.
"Lack of money means DHBs are struggling to replace ageing buildings and outdated alarm systems which have potentially life threatening flaws, not to mention hiring staff and improving safety.
"[Workers are] seeing a steady erosion of their ability to support and stand up for their clients."