Unscrupulous campaigners could buy and sell same-sex marriage survey forms without falling foul of the law, says an electoral expert.
As well, millions of forms could end up at rubbish tips, opening up another opportunity for exploitation.
Former Australian Electoral Commission special adviser Michael Maley - who helped design the Senate electoral system - has outlined a series of problems with the national survey on the Australian Public Law blog.
"At present, there is no legal provision specifically dealing with the possibility of the buying or selling of 'plebisurvey' votes," Mr Maley said.
He noted the Australian Bureau of Statistics, which is running the survey, had said there were "potential penalties for providing false or misleading information".
But even if that applied, it could be virtually impossible to enforce in practice.
"A vote-buying website set up overseas would for all practical purposes be beyond the reach of Australian law," he said.
Mr Maley said the 1997 constitutional convention postal ballot turnout was 46.92 per cent.
A similar turnout for the marriage survey would see more than eight million ballots not used.
"There will be no specific offence of taking them from rubbish bins, or generously offering to collect them for 'recycling'," he said.
The ABS has encouraged voters to destroy survey papers they do not wish to use by tearing them into two or more parts.