The cost of a plebiscite on same-sex marriage is likely to blow out to more than the $160 million already allocated, a Senate committee has been told.
Greg Manning, a senior official with the Attorney-General's Department, told a Senate estimates hearing on Thursday the figure was best described as a "prudent allocation" and "an indicative amount".
Final costings would be determined once the government finalises the model for the plebiscite.
The estimated cost of the plebiscite was also based on the two opposing sides receiving no public funding for their campaigns beyond money for a pamphlet.
It has also been confirmed the government is considering giving money to both sides in the plebiscite, expected to be held this year if the Turnbull government is re-elected, but that no final decision has been made.
The money would come on top of the $160 million allocated in Tuesday's federal budget.
"Were there to be an allocation of public money then of course both sides of the argument would be entitled to equal treatment," Attorney-General George Brandis told the Senate hearing.
"Were there to be public funding, obviously one of the issues would be the appropriate recipients on behalf of the `yes' or the `no' case of public funding."
It's possible, if not likely, that groups such as the Australian Christian Lobby and the Catholic Church, both opposed to gay marriage, would receive taxpayer dollars to argue their case.
Senator Brandis said he had met with representatives of both sides in the debate over gay marriage, as the government continues to work on the final model for the plebiscite.
The attorney-general also confirmed the government did not support calls from advocates for a "no" vote for anti-discrimination laws to be suspended if campaign goes ahead.
"That is not the government's position," he said.
The Australian Electoral Commission later told the committee it had not been asked to do any work in preparation for the plebiscite.
AEC commissioner Tom Rogers said the $160 million put into the contingency fund would have been based on an indicative costing he had given last year of $158.4 million.
Asked whether voting would need to be compulsory, as at a general election, Mr Rogers said: "That would be a choice for government."
