The federal government's economy drive may achieve most of its aims within six weeks, by which time many public service jobs will already be gone.

But public servants seeking a golden handshake will probably be disappointed, as few funds have been set aside for pay-outs.

This week's budget revealed the government expects its non-military workforce to shrink by about 4200 full-time staff in 2012-13, the first significant reduction since the late 1990s.

However, dozens of agencies began preparing for the cuts late last year and have already shed employees.

The budget papers also show the government has revised its estimated spending on redundancy payments downwards for 2012-13, suggesting there will be few pay-outs made after July 1.

The projected spending on separation payments was cut from $43 million to $37 million, even though the budget forecast significant job losses.

Meanwhile, the redundancy bill for this financial year has blown out from an estimated $59 million to $110 million.

Finance Minister Penny Wong said yesterday her department had written to departments to advise them against resorting to forced redundancies. However, several sources told The Canberra Times that demand for voluntary redundancies among public servants was far outstripping supply, which suggests forced retrenchments are extremely unlikely.

The minister has previously said natural attrition - employees retiring or leaving to take up new jobs - will account for most of the staff losses.

The Community and Public Sector Union urged the government last night to ensure no public servant was sacked to meet job-loss targets.

National secretary Nadine Flood warned agencies to comply with the government's advice to avoid forcing staff to leave. ''Our expectation is that there is every opportunity in Canberra to redeploy staff to other agencies, and not resort to forced redundancies.''

Senator Wong's spokeswoman said the government neither set a budget for redundancy payments nor did it forecast how many pay-outs it would make.

The Public Service Commission is expected to publish a six-monthly update of official workforce statistics this month, which should show the extent to which government agencies began downsizing last year.