More than 4000 public servants have voted to go on strike next month.

Photo: RNZ / Richard Tindiller

Union members at Inland Revenue (IRD) and the Ministry of Business, Innovation and Employment (MBIE) voted "overwhelmingly" to take industrial action, the Public Service Association (PSA) said.

Employees at both organisations will take part in two two-hour stoppages on 9 and 23 July.

PSA national secretary Glenn Barclay said union members were "very reluctant" to take this step, but felt they had no choice.

"The level of support in the vote shows the level of frustration," he said.

"MBIE is the agency which advises New Zealand businesses on how to conduct their industrial relations, and we expect better of them.

"Spending millions on contractors while denying our members a cost of living pay increase is poor practice, and it's not in the spirit of the new government's expectations for the public sector."

Meanwhile, union members at Inland Revenue have not taken industrial action in more than 20 years.

PSA national secretary Erin Polaczuk said staff were at the end of their tether.

"Our members take their jobs very seriously and they're trying desperately to keep the system on track while Inland Revenue presses ahead with its Business Transformation project which will see one in three staff let go by 2021," she said.

"To give just one example - IR's system is unable to process the Best Start tax credit, which should give lower income families an extra $60 a week for each child.

"It starts on July 1, and members are told they will have to process this manually, drastically increasing their workload.

"To have all this happening while the employer refuses to consider a modest across-the-board pay increase is deeply upsetting to our members."

Strike notices are being issued to both employers.