The main public sector union said the casualisation of the public service, coupled with the increasing reliance on labour hire and contractors, was a concerning trend.

The Australian Public Service Commission's annual State of the Service report showed the proportion of the public service's ongoing workforce was its lowest since 1993.

The public service had 132,101 ongoing employees at June 30, or 87.8 per cent of the workforce, and 18,373 non-ongoing staff.

Of those, 10,618 were casual staff, accounting for 7.1 per cent of the APS workforce, an increase of 6.3 per cent on the figure in 2001.

The figures, while the continuation of a long-term trend, reflected the government's scrambling to boost its workforce in response to the pandemic.

There were 3809 non-ongoing public servants added to the APS in 2019-20, almost exclusively during the bushfire response and pandemic.

The tax office and Services Australia were largely responsible for those increases, with 1943 and 1242 extra non-ongoing staff respectively.

The Public Service Commission advised all of the non-ongoing roles added during the first six months of 2020 were in service delivery roles.

However, while the boost to public service numbers ends the streak of three consecutive years of contractions, the federal government has flagged this would not continue.

Former Finance Minister Mathias Cormann announced in the October budget Australia's economic recovery would be private sector-centric and the APS staffing cap would remain in place.

Community and Public Sector Union national secretary Melissa Donnelly said the trend could be explained by the government's public service staffing cap.

"This trend to increased casualisation and higher rates of labour hire is not a new phenomenon," she said.

"Scott Morrison has taken this opportunity during the pandemic to double down on this concerning government trend.

"While the government announced new jobs at Centrelink at the start of the pandemic, not one of the 8500 new staff in Centrelink were permanent public sector jobs. Instead they are all casual, labour hire and contracted providers with insecure working arrangements."

Former public service commissioner Andrew Podger agreed the government's reliance on labour hire staff was concerning, but said there were positives to using non-ongoing staff where required.

"I've always thought that the reliance on labour hire and contractors was very silly and reliance on non-ongoing staff was a much more sensible approach to handling shifts in workload," Professor Podger said.

"They come under the Public Service Act so therefore you've got proper merit processes involved in their employment. And secondly, you tend to invest in their training."

The APSC said non-ongoing staff were used to manage seasonal or limited work and had fluctuated between 7 and 11 per cent of the workforce over the previous 20 years.

Data showed between 2015 and this year the proportion of non-ongoing staff in the APS was higher than at any time between 2003 and 2015.

The APSC said increases in the last decade could be partly explained by the movement of casuals at the Australian Electoral Commission and the Australian Sports Anti-Doping Authority to be covered by the APS Act. This also swelled the proportion of APS casuals.

Also the National Disability Insurance Agency has grown from its inception in 2014, with 37 non-ongoing employees, to now include 1214 non-ongoing staff in its ranks.