The cost of the same-sex marriage plebiscite could be $15 million higher than the estimated $160 million if the Turnbull government decides to spend taxpayers' money on the "yes" and "no" campaigns.

Attorney-General George Brandis is preparing a cabinet submission on the mechanics of the plebiscite, which will include a recommendation on whether or not the cases for and against changing the definition of marriage should be publicly funded.

The Australian Christian Lobby, which opposes a change to the definition of marriage, says both the "yes" and "no" campaigns require public funding.

"There's definitely an expectation that there would be public funding of equal amounts," the managing director of the Australian Christian Lobby, Lyle Shelton, told Fairfax Media on Monday.