At the start of a two-month campaign, the confidential research provided to Fairfax Media shows support for a "no" vote has risen, as has the number of people who say they don't know how they will vote.

And alarmingly for "yes" campaigners, turnout could be very low, with just 65 per cent of voters rating themselves as very likely to participate – falling to 58 per cent among those aged 18 to 34.

It prompted a concerned campaign veteran to declare: "There is every chance we can slip behind and lose this."

The research was conducted for the Equality Campaign by Newgate Research pollster Jim Reed between August 28 and September 6, with a sample size of 800 and a 3.5 per cent margin of error.

It showed that 58.4 per cent of those surveyed said they would back a "yes" vote, down six percentage points from two weeks earlier, while support for a "no" vote rose two percentage points to 31.4 per cent. The "unsure" vote rose three percentage points to 10.2 per cent.

The tracking polling, which was conducted before the High Court case verdict was handed down, was released to serve as a wake up call to "yes" campaigners who believe victory is assured. Senior figures from the campaign would not speak on the record about the results.

Campaigners point to the 1999 republic campaign, the Brexit vote and the election of US president Donald Trump as evidence "nothing can be taken for granted".

The "no" campaign has successfully portrayed itself as the underdog in the postal survey, based on years of research showing a majority of Australians support same-sex marriage.

Opponents of change will be buoyed by the significant uptick in voters declaring themselves undecided, a sign that attempts to broaden the debate into areas such as the Safe Schools program may be working.

The intensity of the debate has already forced LGBTI organisations to dig deeper. Switchboard Victoria manager Jo Ball said their support service had taken 30 per cent more calls since the survey was first announced, and recruited 16 additional volunteer counsellors, with more coming.

An angry clash between "yes" and "no" supporters outside a Brisbane church on Thursday night, which saw one arrest and one minor injury, prompted government leaders to again call for respectful debate.

Prime Minister Malcolm Turnbull said there would always be "isolated cases" of unpleasantness and warned: "You cannot expect your side of the argument to be respected unless you respect the other side of the argument and the people who put it."

Finance Minister Mathias Cormann, who is responsible for the postal survey, said it was disappointing that people "take things to extremes" in any political debate.

The major parties will work over the weekend on a bill to ensure normal election safeguards apply to the marriage campaign, including the requirement for material to bear an authorisation.

Labor has pitched for the rules to go further and ban "vile" content, but Senator Cormann appeared to rule that out on Friday, saying: "You don't want to put inappropriate limits on the freedom of political expression."

He was largely backed by the Australian Human Rights Commission, whose representative Ed Santow said people had to take personal responsibility for their conduct in the campaign.

"The law has an important role to play here, but it can only go so far," he told the ABC. He said the government should consider rules limiting speech with "very serious harmful effects", such as incitements to violence.