The bureau confirmed about 90 staff have been removed from projects such as preparations for the 2021 census for work on running the $122 million survey for two months until its completion in November.

There are fears the mass of workers taken off existing projects could lead to delays among the bureau's basic services, as well as its $257 million transformation.

Chief statistician David Kalisch said the majority of the staff working on the survey were based in Canberra, but would not confirm an exact number.

"The ABS has currently reassigned 90 existing staff across different work programs to support the delivery of the Australian Marriage Law Postal Survey. They will return to their positions once their work on the taskforce is complete," he said.

"The $257 million transformation program is not funding any part of the postal survey. The ABS will continue to deliver its statistical and data integration work program, and plan for census 2021."

Deputy Australian statistician Jonathan Palmer told a senate committee into the postal survey on Friday that several people at the bureau responsible for planning the 2021 census had been taken off the project and transferred to work on the survey.

While he said the survey coincided with a wind-down from the most recent census, catch-up work would be required on the 2021 census that would've been carried out over the next two months.

When asked about potential delays to projects at the ABS, Small Business Minister Michael McCormack - the minister in charge of the bureau - referred The Canberra Times to the same statement made by Mr Kalisch.

The survey comes after the bureau suffered multiple job losses during the past year, with up to 100 redundancies at the ABS announced in March on top of the 120 staff axed in 2016.

CPSU deputy secretary Melissa Donnelly said there were fears the postal survey could leave the ABS unable to carry out its basic functions amid the staff cuts.

"This is an agency under massive pressure, being forced to pick up additional work in a short time frame," she said.

"Getting the postal survey done in the time frame the government asked it to be undertaken will require some substantial staffing from the ABS."

Rank-and-file officials at the ABS previously expressed 'grave doubts' about how the bureau would be able to carry out the postal survey in the short time, with some fearing a repeat of last year's census failure.

A survey of 150 CPSU members found the vast majority said the postal survey on same-sex marriage would negatively impact other work programs at the ABS.

Ms Donnelly said the large transformation project could be set back many months.

"It's a hugely important program that's been conducted over a number of years to transform the ABS's infrastructure and processes and to see the ABS is modernised," she said.

"The digital transformation is one of the projects we know that has the risk of being delayed, but across the organisation we know the ABS is under huge pressure to deliver the base-line work, and the postal survey has the potential to impact on other things."

Mr Kalisch did not directly respond to questions put by The Canberra Times about how long the transformation program would be delayed as a result of the postal survey.

Survey forms began to be distributed to eligible participants this week.

The ABS estimates 7.5 million forms were sent out by Friday, or 47 per cent of all forms.