Plans to reform the mental-health system in Australia's most populous state could necessitate replacing hundreds of psychiatrists who are threatening to resign.

Psychiatrists will be joined by nurses and general practitioners on Monday to call on the NSW government to address an "imminent mental health crisis".

About 200 public hospital psychiatrists are planning to resign on Tuesday, while there are already 140 long-term vacancies in the state system.

Premier Chris Minns said the dispute over a sought 25 per cent pay rise would increase psychiatrist salaries by as much as it would cost to hire a first-year nurse.

He blamed the coalition government's wages policy for suppressing public-sector pay for 12 years.

"I can't make that right in a single year," the premier told ABC radio.

The mass-resignation threat was unprecedented, he said, but contingencies had been made in a "state-wide, short-term escalation plan".

The government also had longer-term plans to reform the system, Mr Minns added.

Public patients in mental health distress will be assessed virtually by telehealth psychiatrists as one way of plugging the short-term gap, if authorities can find enough of them.

One semi-retired psychiatrist told AAP they were "not going anywhere near it", despite crisis rates of more than $3000 per shift being offered.

"There might be some people who do but I doubt they will get the numbers they're looking for," the psychiatrist said.

NSW Health secretary Susan Pearce has described locums as a "short-term measure to address what we're facing".

Regional hospitals will be less affected due to their already heavy reliance on contract and temporary doctors.

Lifeline 13 11 14

beyondblue 1300 22 4636