Professionals Australia also says public servants are reporting their agencies are having difficulty retaining technical staff due to both the cap on numbers and the pay freeze that was introduced as part of the pandemic response.

"That's meant fewer people to do the same work and as a result, people are stretched very thinly," the union's director Dale Beasley said.

"It's also resulted in an increased outsourcing of key functions which is both more expensive and results in significant corporate knowledge being lost to the organisation when the contract expires."

A survey of members of Professionals Australia found technical workers wanted to hang on to the flexibility gained as part of the pandemic working-from-home arrangements.

Just over 70 per cent of respondents said they wanted more support to work from home and flexible working arrangements, even outranking the 49 per cent who said they'd prefer higher pay.

Two-thirds said they were more productive working from home, with a great ability to do more complex tasks.

"If my agency does not continue to endorse flexible working arrangements and continue with pay freezes - beyond a reasonable length - this would seriously deter current employees from staying on," one respondent said.

The survey results come just days after the Australian Public Service Commission directed all agencies to send their employees back to the office, with many agencies set to abandon flexibility around working from home.

"People are disheartened by the rush to move straight back to the office," Mr Beasley said.

"Some agencies are doing alright at it and transitioning slowly and embedding ongoing flexibility measures but there are also some really poor examples."

Of those surveyed, 70 per cent said it was getting harder to retain technical professionals before COVID-19 and 63 per cent said public service rates of pay weren't keeping up with the market.

More than half reported issues with keeping in-house capability and respect for contribution of technical professionals.

Billions of dollars of infrastructure announcements will be made on Monday, including $150 million for projects in the ACT, ahead of Tuesday's federal budget. It's a major plank of the economic stimulus the government hopes will lead the country out of recession, but Professionals Australia is warning the expertise needed is in short supply.

"It seems every jurisdiction is looking to turn on infrastructure, technical work, advanced manufacturing," Mr Beasley said.

"It's going to be very hard for every jurisdiction to do that at the same time when the investment in the development of the workforce has been lacking for so long."

The cost of delivering such projects is likely to go up as governments compete for talent, he said. He called for the federal government to remove the average staffing level cap on the public service to help with recruitment.

"The ASL cap should have gone a long time ago, but it should definitely go now," Mr Beasley said.

"What the government needs to do is work out the workforce it needs to deliver the essential projects that are going to be on the books and to attract and retain that workforce it's not going to be of any help to have some arbitrary ceiling on the number of public servants.

"The government should be looking at public service employment as part of the economic stimulus puzzle."

It is still to be seen what Tuesday's budget will hold for the public service, its bargaining policy and the average staffing level cap.

Before the pandemic hit and the budget was set to be balanced, the government had signalled it would consider the future of the cap, which it said must be in place while the budget was in deficit.

But now the budget is set to stay in deficit for years, until unemployment drops below 6 per cent.