Tony Abbott's public ballot on same-sex marriage, now embraced by Prime Minister Malcolm Turnbull, will cause major social harm and cost the economy more than $500 million in direct expenses and lost production.
The publicly funded for-and-against campaigns, which some have argued should be given special dispensation to transgress hate-speech laws, will cost at least $6 per voter based on overseas experience.
That's the conclusion of modelling undertaken by one of the world's most respected business accountancy consulting firms, PricewaterhouseCoopers Australia. 
In a serious blow to the claimed legitimacy-building power of the proposed plebiscite, PwC Australia has examined the proposal and warned it will do more harm than good, leading to high levels of social tension, discrimination, mental health and mood disorders, and will cost taxpayers and business far more than previously understood.
It found the plebiscite itself would cost $158 million to stage, not counting the extra $66 million in public funding likely to be committed to promote both the "yes" and "no" arguments. The modelling suggests there would be another $281 million surrendered from the national economy from lost production as people take the time to vote. That is, around $525 million all up.
"The real costs to government, the economy and members of the community to hold a stand-alone plebiscite are more than three times higher than the numbers commonly quoted," said PwC Australia chief executive officer Luke Sayers.
"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, whose organisation was ranked Australia's top LGBTI (lesbian, gay, bi-sexual, transsexual, inter-sex) employer last year in the Pride in Diversity Australian Workplace Equality Index, described the plebiscite as "a massive waste of time and money, that will remove focus on the economy, growth, and jobs, which is the real priority for Australia".
"The mechanism chosen to make this change is vital to minimise the cost to the economy and health and wellbeing of our communities," he said.
"Our modelling points to a parliamentary vote as the best mechanism for change." 
That view aligns with what Mr Turnbull used to think before he did a behind-closed doors deal with hardline social conservatives in his party to adopt the Abbott model.
Last year, before taking over as leader, he was steadfastly opposed to the plebiscite, regarding it as unnecessary because there is no constitutional amendment needed. He also argued it would be socially divisive and would become a political headache for the Coalition government unless dealt with prior to the election via a simple free parliamentary vote.
As Fairfax Media reported exclusively in   January, marriage equality campaigners have kept close tabs on the privately held positions of all 226 federal legislators and are now confident that the required majority would be present in both houses were each member freed of party or cabinet restrictions.
Fairfax Media's independent checks with non-declared MPs, and with a small number who have quietly switched to become backers of change in any free vote, supports that conclusion.
Australian Marriage Equality national director Rodney Croome said the result is a wake-up call, arguing "every single government minister must read this report before cabinet considers legislation for a plebiscite".
"The more information we get about the plebiscite, the clearer it becomes that it's just an incredibly costly and harmful opinion poll," he said.
"It's a sign of maturity and strength to change your mind in the face of new information. The plebiscite legislation will be a minefield of unexpected costs, unintended consequences, and complications about timing and public funding.
"More and more Coalition members are speaking out against a plebiscite and I encourage them to raise the issue in the party room as soon as possible."
The modelling assumes the costs would be incurred over a two year period from   January this year, and that the plebiscite would be binding.
Attorney-General George Brandis is understood to be close to taking a proposal for a plebiscite to cabinet, although the government has stepped away from his recent declaration that it would be held this year.
PwC undertook the modelling of its own volition having decided to join the more than 800 organisations that have signed a corporate letter backing marriage equality.  
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