But the public sector union says the move is a hiring freeze in disguise that will hit jobs and services in regional Australia and the bush.

Public service departments have been told by the Abbott government they can hire new staff but only for ''critical'' or specialist jobs as part of ''interim arrangements'' for the service.

Public Service Minister Eric Abetz's office says he has moved to clear up the confusion surrounding hiring practices that has dogged the bureaucracy since the new government took office in September pledging to cut 12,000 public service jobs through natural attrition.

Some departments have imposed partial hiring freezes in the wake of the new government, while others have ordered that any decision on new jobs had to be approved at the very top.

Senator Abetz announced that agencies must now demonstrate they have given existing staff every opportunity to apply for vacant jobs before hiring outsiders.

Agency and departmental heads were expected to brief staff on Thursday on the new guidelines while the Public Service Commission works out the technical details of the ministerial order.

Senator Abetz said that specialist and temporary jobs could also be filled from outside the service.

''The new arrangements will maximise the use of capability already within the APS by increasing efforts to provide every employment opportunity possible to existing staff,'' the minister said.

''External advertising of APS vacancies and non-ongoing temporary employment will be limited to critical vacancies only, most particularly in specialist roles, where a suitable employee cannot be found within the APS.''

Indigenous and graduate recruitment programs would also continue, he said.

''The government will ensure that these arrangements will not compromise the ability of the APS to continue to support the government and the Australian public to the highest standard,'' Senator Abetz said.

CPSU national secretary Nadine Flood said small-town Australia would feel the pain of Mr Abetz's decision.

''This decision is particularly tough on regional communities which have only a limited number of public sector employers in town,'' Ms Flood said.

''If someone loses their job and can't be redeployed locally, they'll be forced to leave town to find other work.

''We call on the government to change this new policy to allow agencies to fill vacant positions locally and guarantee regional jobs are not lost.''

The union leader said the government's move would close off career options in the public service and cause delays and backlogs for vital services.

''Public sector jobs are sought after by many Australians, particularly in regional Australia. This decision is bad news for the many people looking to join the public service.

Canberra Labor MPs Andrew Leigh and Gai Brodtmann, and ACT senator Kate Lundy, released a joint statement calling it "a simple-minded, blanket approach to the public service, with no consideration for demand, efficiency or specialisation".