The Turnbull government's same-sex marriage postal survey will be vulnerable to fraud and need to include a "voice" option for people who can't be reached by mail, the Australian Bureau of Statistics has indicated.
Several shortcomings of the $122 million non-compulsory, non-binding opinion poll were laid bare at an inquiry fronted by the under-pressure ABS on Thursday, as a legal challenge continued to wind its way through the courts.
The ABS conceded it could not stop people filling out other people's ballots, and indicated its fraud prevention program would rely heavily on individuals reporting their survey forms as missing.
Deputy statistician Jonathan Palmer was also unable to say how the ABS would ensure the survey would reach thousands of Australians in remote communities, but promised a "paperless" option that would let people vote by phone, text or online.
It was revealed the ABS, which has been tasked with surveying the Australian population's views on whether same-sex marriage should be legalised, will spend $6 million on the process before the High Court decides next month whether the survey is legally valid.
And Mr Palmer confirmed the ABS was only informed about its role in the postal survey the day before it was announced by the Turnbull government. Nonetheless, he assured Australians the ABS would not repeat the debacle of the 2016 census.
"We've reflected hard on the lessons learnt," he said. "We failed to manage some key risks ... we've improved our risk management practices and we're taking that learning into this exercise."
Senators inquired as to how the ABS would protect against fraud in the inevitable event that forms were sent to a voter's old mailing address and filled in by another person.
"It would be against the law to open someone else's mail. There would also be penalties around misleading the statistician," Mr Palmer said.
But he warned: "You can't guarantee against theft in a postal process."
If the form's legitimate recipient contacted the ABS to say they had not received their survey, it would be cancelled and a new form dispatched, Mr Palmer said.
"That's why we're advertising for people to update their details on the electoral roll," divisional manager Samantha Palmer added.
Asked about how the ABS would reach Indigenous Australians in remote communities, Mr Palmer said he was "not in a position to today to tell you all the details of [the] inclusion strategy for that important community". But "it will include a voice option", he said.
As well as preparing a national tally of opinions and electorate-by-electorate breakdowns, the ABS will publish demographic information on the people who responded. The information, broken down by age, sex and electorate, will enable assessments of how representative the responses were.
Fairfax Media understands the breakdown was insisted on by the minister in charge of the survey, Mathias Cormann, who wants no doubt left about the representativeness of the $122 million survey.
At Thursday's inquiry, Mr Palmer confirmed the ABS would not adjust the tally based on the non-response rate for various demographics. Same-sex marriage advocates fear young people will be less likely to respond.
The ABS defended its ability to conduct the poll, rather than the Australian Electoral Commission, because this was the collection of statistical information and not a vote.
On several occasions Mr Palmer corrected senators use of the term "ballots", insisting they should be called "survey forms". He also dissuaded people from including extraneous material with their forms, warning glitter would be "particularly problematic" for the machines that process the forms.
Meanwhile, High Court Chief Justice Susan Kiefel said a legal challenge led by Tasmanian MP Andrew Wilkie would be heard "hopefully together" with a separate case brought by national organisation Australian Marriage Equality.
The AME case raises separate but related legal issues.
The High Court, which was sitting in Brisbane but had parties appearing via videolink in Sydney and Melbourne, heard the Commonwealth government will attack the parties' "standing" or legal capacity to bring the challenges.
