The High Court will hear a challenge to the same-sex marriage poll just a week before survey forms are due to be mailed.

The hearing, scheduled for September 5 and 6, comes with voters, activist groups and political parties all gearing up for the ballot.

Two marriage equality groups took their case to overturn the ballot on constitutional grounds to Justice Susan Kiefel on Friday.

No forms will be distributed before the case is determined.

"Just to reach this stage for the full bench to agree they will hear this matter is a win for all of those people in the community who have all sorts of concerns about this non-binding, voluntary postal vote," said independent MP Andrew Wilkie, one of the applicants.

Despite the potential delay, the Australian Electoral Commission has been deluged with 17 times its usual daily intake of electoral roll updates.

In a significant boost to the "yes" campaign, GetUp revealed it would work with marriage equality groups to run a "powerful grassroots campaign" but would also support one of the High Court challenges.

"If we are forced to a postal survey on marriage equality, GetUp will defer to the results of a recent survey conducted of the LGBTIQ community ... which indicates the community does not support a boycott, and thus engage in a campaign to win a 'Yes' result," campaign director Sally Rugg said.

GetUp is backing the Human Rights Law Centre legal challenge and will fundraise from its membership.

A second challenge is being run by the Public Interest Advocacy Centre.

Qantas has also thrown its weight behind the "yes" case.

Opposition Leader Bill Shorten has urged supporters of same-sex marriage to get behind the "yes" case, despite his ongoing concerns about the $122 million "dog's breakfast" postal ballot.

He has written to Malcolm Turnbull inviting the prime minister to co-sign a letter to every Australian in support of a "yes" vote.

Mr Turnbull was circumspect about Mr Shorten's call for a joint letter, again criticising Labor's decision to block the February plebiscite.

"I've been a supporter of gay marriage much longer than him," he told 3AW radio.

"Whether I sign a letter with Bill Shorten, I'll reflect on whether that is useful. It may actually be counter-productive."

Meanwhile, the minister in charge of the ABS, Michael McCormack, has apologised for an editorial he wrote almost 25 years ago.

"A week never goes by any more that homosexuals and their sordid behaviour don't become further entrenched in society," he wrote in a 1993 regional newspaper editorial.

Mr McCormack said he had grown and learnt not only to tolerate but to accept all people regardless of their sexual orientation.

Labor senator Jenny McAllister, who next week will seek an inquiry into the ballot process, says it is not only expensive and divisive but appears to have "no integrity, no protection from misleading and deceptive material, and will not be protected against ballot fraud, bribery and interference".

Finance Minister Mathias Cormann says the government is open to considering legislation to allow for the usual election rules to apply to the survey.

It is also looking to fix a potential problem with "silent" voters and 16 and 17 year olds on the electoral roll.

Forms are due to be posted to voters by September 12 and completed by November 7, with a result to be announced on November 15.