Human resources specialist NGA.NET is warning of a "retirement tsunami", with 44 per cent of Australia's public sector workforce eligible for retirement within the next decade.

The firm says departmental bosses need to apply lessons they learned during the 18 month hiring freeze, which officially ended on Wednesday, if they are to cope with the coming exodus of their over-55s.

Managing Director Karen Evans says public service hiring managers are going to need all the skills they learned during the hiring freeze if they are going to compete for a dwindling pool of experienced workers.

The Commonwealth's 55-plus brigade is the only age group in the bureaucracy that is growing, according to the latest figures, while only 2.5 per cent of our federal public servants are under 25, down from 5 per cent in 2007.

Meanwhile the proportion of employees 50 years of age and over has also grown strongly over time, increasing from 20 per cent of all ongoing employees at June 2000 to 31.4 per cent at June 2014

Ms Evans said on Wednesday that more and more of her clients were complaining of shortages of workers in the more experienced cohorts.

"Forty four per cent of Australia's public sector workforce will hit retirement age within a decade," she said.

"We know from our customers that there is a need to keep that depth of experience that you get from retirement-aged people.

"A lot of them are coming back on contract even after retirement."

In the 18 months of the hiring freeze, public service bosses developed a number of techniques for getting around the recruitment restrictions imposed by the Abbott Government and Ms Evans said that experience in thinking outside the square should hold them in good stead as they confront the growing retirement challenge.

"The interim recruitment arrangements had a benefit," she said.

"It was challenging, but I think that some of the innovations that departments put in place had a real impact on engagement in the workforce and engaging what you've got clearly becomes more important in the war for people as there is less workforce to capture.

"It's going to become huge, the competition for those people who are good and are available will become enormous."

There will be a greater focus in departments on holding on to their good performers in a tougher recruiting environment, Ms Evans said.

"The things that are still so important today, like understanding what you have in your workforce, being sure that you got development plans in place for them to succeed into roles," she said.

"With the retirement tsunami, it is even more important to make sure you are actively planning to transfer knowledge, engage you workforce.

"That means people are more productive, you've got less absenteeism, people are going to stay with you but it also means that the word is on the street that you're a great place to work."