Fresh figures show workers are doing more overtime, many for no extra pay.
The 2015 Hays Salary Guide reveals 29 percent of firms increased workers' overtime over the past year, more than a third of them by up to 10 hours a week.
The findings are based on a survey of 451 employers, covering 374,000 members of staff.
Just over half of employers reported salaried staff were not paid extra any hours beyond the standard 40 a week.
Managing director of the recruitment agency Hays New Zealand Jason Walker said highly skilled professionals on salaries were doing a lot of the extra work.
He said 76 percent of employers expected a surge in business over the coming year, and firms should consider employing more staff to ease the pressure on workers.