Australian Bureau of Statistics insiders have raised "grave doubts" that the same-sex marriage postal vote can be done properly.

ABS workers have told reporters that the postal survey will be a “rush job” that could go the way of last year's Census debacle, while others worry it will not be representative of Australians’ opinions of same sex marriage.

Chief Statistician David Kalisch insists the ABS is up to the task, though he admits the deadline of November 15 is “challenging”.

Mr Kalisch has confirmed that the Department of Human Services is being tasked with running a plebiscite call centre, and that the ABS has had to set up an internal task force specifically to face the challenge of conducting an accurate postal survey.

The Community and Public Sector Union (CPSU) says that a recent survey of its members at the ABS found 74 per cent did not believe the bureau could handle the job.

The survey covered less than 5 per cent of the total number of ABS staff.

Over 40 per cent of respondents had no confidence the plebiscite could be properly administered in the allotted time-frame, while another 38 per cent were not sure.

The union members said running the poll would “negatively impact on other work programs” because the ABS does not have enough staff.

“It will be a big rush job and the ABS has shed too many staff in recent times since the Census, so I don't think the agency will be up to the job in such a tight timeframe,” one ABS employee told Fairfax reporters.

“A large, high-profile, heavily politicised, controversial project, outside our sphere of expertise, to be done in a very short timeframe using methods known to be statistically unsound, outside the legal framework that normally covers voting in Australia,’ another worker alleged.

“How can this possibly end well?”

Mr Kalisch told staff he rejects media “aspersions on the integrity, professionalism and capacity of the ABS”.

“This is our opportunity to show what the ABS can deliver,” the Chief Statistician wrote to ABS staff.

“We will deliver quality statistics back to the community about their views on marriage law.”

The CPSU says the grave doubts held by bureau staff cannot be ignored.

“ABS staff are highly qualified professionals and know full well what work the agency should and shouldn't be doing,” the union's Deputy Secretary Melissa Donnelly said on Wednesday.

“This survey makes it clear that many of them have grave reservations about whether the ABS is the right agency to run the postal plebiscite.

“The Turnbull Government should listen to these concerns.”