Already-strained government services could be stretched to breaking point if Treasurer Joe Hockey cuts thousands of public servants, their union warns.

The federal budget is expected to reveal how many of their jobs will be slashed.

The coalition went to the 2013 election with plans to cut 12,000 jobs through natural attrition.

But Mr Hockey had to put those savings on hold after discovering the former Labor government had already put in train a scheme to shed 14,500 jobs.

The public sector union now says workers are worried as many as 26,500 jobs could go from outsourcing, asset sell-offs and agency and office closures.

Its fears have been heightened by the government’s commission of audit, which recommended the public service be trimmed by 15,000.

Agencies such as the Australian Quarantine and Inspection Service, Customs and the CSIRO already have experienced job losses, the Community and Public Sector Union says.

National secretary Nadine Flood estimates 6000 jobs have been cut ahead of the May 13 budget, and public services are groaning under the strain.

“The rivets are starting to pop,” she told AAP.

Mr Hockey has previously said he would detail the coalition’s plan for the public sector after considering the recommendations of the commission of audit.