The union representing public servants has slammed the decision to hand responsibility for the government’s same-sex marriage postal plebiscite to the Australian Bureau of Statistics.

The Community and Public Sector Union, which represents many of the public servants working at the ABS, says the agency has faced constant job cuts and demands from government.

“We have serious concerns about the capacity of the ABS to run a postal plebiscite,” CPSU Deputy Secretary Melissa Donnelly told SBS World News.

“This is an agency already under massive pressure and struggling with its core functions after two rounds of job cuts.”

The government is planning to decide whether to legalise same-sex marriage either by an in-person or postal plebiscite this year.

It will shortly introduce legislation for a compulsory national ballot to take place on November 25.

That move will almost certainly fail, opening the way for the government’s second option of a voluntary postal plebiscite. A final result would be known no later than November 15.

Finance Minister Mathias Cormann told reporters in Canberra he has the power to hand the organisation "up to $295 million" to do the job.

“That's more than enough to be able to fund the postal plebiscite should the senate not support the government's preferred scenario … of a compulsory attendance plebiscite,” Minister Cormann said.

The ABS recently oversaw the bungled 2016 Census response and flagged concerns with a senate inquiry into it over the pressures it was facing.

“While the ABS has strived to operate efficiently and has a strong track record of innovation to improve its efficiency, the ABS has occasionally had to reduce its work program to remain within its budget and to continue to produce high quality statistics,” the agency said in its submission to the inquiry last September.

“Most recently this occurred in 2014, when the ABS discontinued six statistical programs and made reductions to a further seven.”

The agency also said it would have to reduce non-Census staff by 740 people over the next three years.

“It’s entirely appropriate and indeed will be essential for the ABS to be provided additional funding and resourcing if it is to run this postal plebiscite, but the fact remains that the bureau is clearly not the most appropriate agency or organisation to be carrying out this task,” Ms Donnelly said.

“It appears once again that political expediency rather than logic is driving the decision making process.”

Minister Cormann told SBS World News he had "full confidence" in the ABS's ability.

"The ABS will be provided with the appropriate resources to conduct this postal plebiscite process should that be required if the senate decides not to support the Bill for a compulsory plebiscite."