The Community and Public Sector Union is hoping to change that with the launch of its own census for labour hire staff on Wednesday.

The Australian Public Service Commission conducts its own annual survey each year to collect the attitudes and opinions of the 150,000 federal public servants on a range of subjects, including management, workplace conditions, inclusion, diversity, discrimination and corruption.

But labour-hire staff and contractors are not included in that census.

The union's national secretary Melissa Donnelly said it was time to get the perspectives and attitudes for the more than 20,000 workers it believes are currently employed on contracts within the public sector.

"[The union's census] will count the APS labour hire workers that the government refuses to," Ms Donnelly said.

The census launch comes as new figures provided to The Canberra Times show an average of nearly one-fifth of department workforces are made up of labour hire or contracted employees with some departments, such as Veterans' Affairs, recording more than 40 per cent of non-APS staff.

Ms Donnelly said the high percentage of contracted workers was a result of a policy implemented by government to tie public service staffing levels to those during the Howard era.

It's a tactic Ms Donnelly believed was a way of "stealthily privatising" the sector.

"Every year the government counts its staff, and seeks feedback from its workforce, but due to the [staffing] cap tens of thousands of workers that deliver for the public sector are left without a voice, ignored by the government and the APSC," Ms Donnelly said.

"The government is trying to privatise the public sector by stealth, and at the same time is refusing to acknowledge or hear from the workers they rely on."

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While estimates range in the vicinity of 20,000, the true figure of labour-hire and contracted employees within the public sector remains unknown.

The commission, which collects key data on the sector among other tasks, has previously said it doesn't record that figure.

In an answer to a question on notice by Senator Katy Gallagher during Senate estimates, the commission said it didn't collect data on contractors and labour hire engaged by the agencies as they were not employed under the Public Service Act 1999.

The use of various workforce firms means some contractors are not bound by the same terms and conditions expected of regular public service staff, including the APS Code of Conduct.