A rising population is partly to blame for mental health nurses having to work double shifts, says the Public Service Association.
The Waitemata District Health Board is to close eight beds at He Puna Waiora acute mental health unit because it is short of 12 nursing staff.
Union organiser Brendon Lane said that was because nurses were getting burnt out and they were looking for work elsewhere.
"We've noticed across the region that our members are having to work longer and longer hours, more and more often to cover gaps in the roster because of vacancies, sickness and they're just getting burnt out."
Mr Lane said immigration was a factor in rising populations with about 90,000 people settling in the Waitemata DHB area in the past 18 months.
He said the number of people working up to 16 hours a day had increased over the last couple of years and that was a concern because it affected the ability of those nurses to properly care for patients.
"We know that if the numbers and the ratios in the wards are not at a safe level, then the care that they're getting there is not the best care that we could possibly give them."
Mr Lane said numbers were rising in other health board areas and the government needed to step up.