Psychologists employed by District Health Boards (DHBs) will begin refusing overtime on Wednesday as part of a five-week strike.
A nationwide ban on all overtime is in place after claims DHBs failed to present an offer in bargaining last week.
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Association of Professional and Executive Employees (APEX) national secretary Dr Deborah Powell told Newshub psychologists are working long hours.
"The number of psychologists we have is utterly inadequate and so they're taking on as many patients as they can and then inevitably results in some overtime."
That number is getting worse, despite the clear need for more.
"We know that we need roughly 300 more clinical psychologists in health as a result of the mental health commission's findings, but we're losing about 70 a year," Dr Powell said.
She said the working conditions make it extremely difficult for psychologists to keep it together.
"If you have to do too much it falls apart like a pack of cards to be entirely honest, you just can't do that and do your job."
The strike runs until September 3.
Auckland DHB said in a statement announcing the strike it respects the psychologists right to strike and know it is a very difficult decision to make.
A spokesperson for the DHBs told Stuff they have contingency plans available to manage the strike and are continuing to work on the issues identified by the union.