A last-minute surge has seen a record number of Australians enrolled to vote ahead of the upcoming postal survey on same-sex marriage.

The Australian Electoral Commission on Friday released figures it described as "extraordinary" that show 90,000 new voters - mostly the young - had joined the roll since the survey was announced on August 8.

The development delighted same-sex marriage advocates but alarmed nervous Coalition MPs, who believe the enlarged youth vote will come back to bite them at the next election.

A further 675,000 people had updated their electoral details, and 165,000 transactions were yet to be processed as of Thursday night, the AEC said. In total, almost 1 million Australians had either enrolled for the first time or updated their details.