Photo: William West/ AFP/ Getty Images.

Betting markets for Australia’s national postal vote on marriage equality still favour a positive outcome, but odds are shortening on a shock No vote that would set the cause of same-sex marriage back for years.

Sportsbet, which controversially opened a betting market on the outcome of postal vote last weekend, has now taken thousands of dollars in bets on the outcome.

The number of people betting on No has been running three to one against a Yes outcome, a Sportsbet spokesperson told Business Insider.

With some marriage equality advocates remaining opposed to the the $120 million postal vote as a means of resolving the issue, odds on a No vote have fallen from $3.00 on the weekend to $2.25 today.

The odds of a Yes vote have lengthened, from $1.30 to $1.60 today.

Sportsbet spokesman Christian Jantzen told Business Insider that the bookies misread the likely outcome with its initial prices.

“Clearly we were of the opinion that it would be a done deal,” he said. “All in all it’s looking likelier to be a tight result.”

Every Australian registered on the electoral roll will get a ballot paper next month, to be returned by November 7th. A result will be declared by November 25th. A Yes vote would clear the way for a free vote in parliament within weeks to legalise same-sex marriages.

While polls have shown for years that the community is broadly in favour of allowing same-sex marriage, the absence of compulsory voting and divisions in the Yes campaign mean the result is not yet a foregone conclusion.

The Yes campaign is split on its support for the postal vote, with long-standing advocates for marriage equality mounting a High Court challenge to the survey, questioning its legitimacy.

The full bench of the High Court will hear the case in early September, a week before ballot papers are scheduled to be sent to households around the country.

Opposition leader Bill Shorten and the Labor Party have reluctantly decided to campaign in favour of a Yes vote.

Shorten has been pleading with marriage equality advocates to drop their resistance to a postal ballot, with legal challenges threatening to delay or completely derail the current path to changes to the Marriage Act.

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