The federal government will have spent well over $6 million before the High Court gets to decide whether it has the authority to spend taxpayers' money on the same-sex marriage poll.
The revelation came as the High Court confirmed it would hear the challenge against the constitutionality of the $122 million survey in Melbourne on September 5-6.
Jonathan Palmer, the deputy Australian Statistician in charge of the survey of 16 million voters, told a Senate inquiry a task force of 40 people had been formed to design the survey.
The inquiry heard that by September 12 - the date promised to the court by the Australian Bureau of Statistics to hold off on sending out the survey forms - it would have spent $6 million on advertising.
Finance Minister Mathias Cormann on Thursday exempted the advertising campaign from scrutiny by the independent communications committee.
However the ABS would still be "required to comply with the underlying principles of the guidelines" and other rules, he said.
The ABS will soon enter into a print and dispatch contract, but this would have an "opt-out date" if the survey does not go ahead, Mr Palmer said.
However, he did not provide details on the financial aspects of the contract.
There were already 250 staff working in a Department of Human Services information centre, taking about 1000 calls a day in regard to the survey, which would expand to 750 workers.
And at its peak "several hundred" ABS staff would be involved.
Responding to concerns the ABS had botched the census, Mr Palmer said lessons learned from it would be used to run the survey.
"That was clearly a case where we failed to manage some key risks ... (and) we are taking that learning into this exercise," he said.
One of the key risks would be reaching older Australians, people with disabilities, remote indigenous communities and overseas voters, which the ABS would deal with through an "inclusion strategy".
Mr Palmer said voters could be confident their responses would be kept secret.
Another risk was a low rate of return of surveys, but Mr Palmer would not comment on what would be an acceptable rate of return to make it successful.
Addressing the issue of glitter being used on survey forms or in envelopes, Mr Palmer said he would be instructing Australians to use a dark pen or pencil and not include any "extraneous material" in the return envelopes.
"I think glitter is particularly problematic because these forms have to go through a scanning machine and I'm pretty sure they are not robust to lots of glitter," he said.
Asked what would happen to a glitter-decorated form, he said: "They would subject to some special process, so the forms might be either cleaned or transcribed but we won't be pumping them through scanners and have them clog up."