A stand-alone plebiscite on same-sex marriage could cost the Australian economy more than half a billion dollars, according to new research.

Key points: PwC modelling finds gay marriage plebiscite would result in $280m in lost productivity

PwC modelling finds gay marriage plebiscite would result in $280m in lost productivity That cost comes on top of $158m for nationwide campaign

That cost comes on top of $158m for nationwide campaign Other costs included impact on mental health and wellbeing of LGBTI community

Other costs included impact on mental health and wellbeing of LGBTI community Cost would be reduced to $113m if the plebiscite was held at same time as election

Modelling undertaken by accounting firm PwC says it would result in $280 million of lost productivity, on top of the cost of running the nationwide campaign, estimated at $158 million by the Australian Electoral Commission.

The cost to the community of funding the two campaigns for and against the proposal would cost $66 million, while the impact on the mental health and wellbeing of the gay and lesbian community amounts to $20 million.

PwC Australia chief executive Luke Sayers said the real cost of the plebiscite was three times what has been quoted by the Federal Government.

"Total economic costs have not been considered before and should be part of the debate on the best way to achieve a resolution to this issue," Mr Sayers said.

The report by PwC, which was commissioned in-house, looked at three different scenarios for approaching the issue.

One involved a stand-alone plebiscite, while another looked at holding the vote at the same time as the federal election. A third option examined the financial impact of a vote in federal Parliament.

The overall cost of the vote would be reduced to $113 million if the plebiscite was held at the same time as the federal election, dropping to $17 million dollars if a vote was held in Parliament to resolve the issue.

PwC economics and policy partner Jeremy Thorpe said they analysed the impact of similar votes overseas.

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"Overseas examples show that spending on the 'for' and 'against' campaigns alone can reach over $6 per voter, as happened in California," Mr Thorpe said.

"That's a huge waste of money that could be better allocated in our low-growth economy."

The report stressed while there is limited data on the on the impact of mental health of Australian LGBTI people with a focus on discrimination, other studies were able to quantify the impact of mood and anxiety disorders.

It estimated 50,000 people — or about 5 per cent of the LGBTI community — would be negatively affected if a stand-alone plebiscite was conducted.

"What we find is the cost to the LGBTI community are in the order of $20 million for dealing with additional mental health costs that arise, just because of the stress and the public nature of the plebiscite," Mr Thorpe said.

The figure stems from the estimated cost to the health system and the loss of productivity in the workplace.

The report said the estimated impact on mental health is "conservative" compared to data from local front-line services, which have reported a sharp increase in requests for assistance since 2014.

It noted while those with strong beliefs against same-sex marriage could also be impacted by the plebiscite, there was "limited evidence" publicly available to the researchers.

PwC undertook the modelling of its own volition, having decided to join more than 800 organisations that have signed a corporate letter backing same-sex marriage.

Prime Minister Malcolm Turnbull has vowed to push ahead with a plebiscite, following debate in the Coalition partyroom last year.

The Federal Opposition and the Greens say the issue should be resolved by a vote in Parliament, instead.

PwC report 'skewed', say ACL and Eric Abetz

In a statement, the Australian Christian Lobby criticised the report and PwC, saying the figures lacked objective credibility.

"[PwC] is a public supporter of same-sex marriage and of Australian Marriage Equality, who are working on the yes case for the plebiscite," a spokesman for ACL said.

"Having voiced such strong public support for same-sex marriage, PwC is not an unbiased source of information when it comes to the plebiscite or in efforts to redefine marriage."

Liberal senator Eric Abetz described the report as "skewed" and questioned how PwC calculated the cost of lost productivity as a result of holding the plebiscite on a Saturday.

"Because, quite frankly, to try to take into account and double the figure on the basis of the time taken to vote as being a factor basically says, 'look, let's get rid of democracy; each time people go to the polling booth it costs them $281 million'," he said.

"And of course it doesn't take into account the costs on the other side of this debate — people who feel strongly, who will feel depressed about a change if it were to occur as a result of a plebiscite."

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