The government has confirmed that departments have been given the go-ahead to demand longer hours from their workers in return for annual pay rises.

Unions say working hours are being raised by more departments and agencies as part of service-wide enterprise bargaining talks and that more time at their desks is a possibility for many Commonwealth employees.

The government has ruled there will be no pay increases in the bureaucracy without "genuine productivity gains" and some departments are telling workers that could mean working longer hours.

The office of Public Service Minister Eric Abetz confirmed this week that departments were free to demand more working hours and the Public Service Commission said longer days could be exchanged for pay rises.

The Professionals Australia union said the move was a return to 1950s-style workplaces.

It said public servants were already working hundreds of hours in unpaid overtime to make sure jobs were done as budgets and workforces dwindled.

Standard working hours vary across the public service but are usually between 36.75 and 37.5 hours a week.

National Employment Standards rule that maximum full-time weekly hours are 38 hours plus reasonable additional hours.

But workers at the Australian Bureau of Meteorology were told this week their work hours could be a key target as bosses seek elusive productivity gains.

"An example of a genuine productivity gain is an increase in the ordinary weekly hours of work," the bureau's deputy director, Vicki Middleton, wrote to her staff.

Senator Abetz's office said workplace agreements would be negotiated at agency level and confirmed the government approved of departments offering longer hours for pay rises.

"In certain circumstances extra hours is one of a number of options available to agencies to achieve productivity offsets," a spokeswoman for the minister told Fairfax.

Every deal stuck between a department and its workforce has to be approved by Senator Abetz under the government's bargaining policy.

The Public Service Commission, the agency co-ordinating the service's stance, was more detailed.

"The government's bargaining policy provides that agencies cannot reduce current hours of work," a spokeswoman said.

"There is nothing that prevents agencies from negotiating to increase hours, having regard to the National Employment Standards, for example as a productivity offset for wage increases."

Professionals Australia public service organiser Dave Smith accused the government and public service chiefs of a "punch-clock mentality''.

"This is a double whammy," Mr Smith said. "At the same time as jobs are being cut, employees are effectively looking at working an extra week every year.

"This is a 1950s approach that focuses on a ''punch-clock'' mentality rather than outputs. It's not a genuine productivity.

"Our members are already working hundreds of additional unpaid hours to cover existing science and engineering skills gaps."