Almost half of all Australians eligible to take part in the same-sex marriage postal survey will receive ballots before the weekend.
More than four million forms have been delivered and millions more are expected to land in letterboxes on Friday, a Senate inquiry has been told.
"We're slightly ahead of our schedule for dispatch of remaining forms, so that all means we're in very good shapes to all eligible Australians by September 25," Deputy Australian Statistician Jonathon Palmer said.
Mr Palmer expects all forms to be sent out by September 21.
Initial mailouts were targeted at rural and remote communities and radio ads for the postal survey are running in seven different indigenous languages.
Roughly 120,000 silent electors are expected to receive their ballot papers next week.
However overseas voters will not receive online codes to take part in the survey until September 25.
"We're not bringing that option online yet," Mr Palmer told senators.
People will not be notified if their returned forms are discarded because they are damaged, marked unclearly or deemed a duplicate.
"Their response is de-identified so there's no receipt or communication to people if there response isn't going to be processed," Mr Palmer said.
External observers - unlike scrutineers used in election campaigns - will be barred from looking at every survey form.
"We don't want anyone in the process to be in a position to estimate the result ... or be seen to be credible in their estimation of the result," Mr Palmer said.
"We'll keep the statistics under strict embargo and very few people - I'm talking less than a handful - will know the result before it's published."
Mr Palmer referred some incidents of alleged fraud, such as the sale of ballot papers on eBay, to federal police on Thursday.
He hopes to discuss with the AFP on Friday a policy for how the two agencies will work together on allegations.
More than 87,000 calls have been made to a hotline set up to assist Australians with queries about the postal survey, with four per cent listed as complaints.
One per cent were classified as a compliment.
"We'd like that number to be higher," Mr Palmer conceded jokingly.
The bureau has had to reassign staff working on the next Census project in 2021 to the survey.
So far the ABS has spent $63.5 million on the ballot.
So far 61 survey forms have been reissued to replaced spoilt ones - including those with drawings by small children.
