It is an issue that has divided the Coalition. After a marathon, seven-hour partyroom meeting in August last year, the Liberal and National parties voted to take the issue of legalising same-sex marriage to a national poll.

Since then, it has become the proverbial political hot potato. And an expensive one at that, costing about $160 million.

That has angered many advocates of same-sex marriage, who argue the debate over the merits of allowing same-sex couples to marry will be full of vitriol and will denigrate their relationships.

Proponents of the status quo maintain such a landmark change to the definition of marriage should be put to the people, rather than being decided within the Parliament.

Prime Minister Malcolm Turnbull took a plebiscite to the election — a free vote in Parliament is not on the cards.

How do we get to a plebiscite?

Politicians first need to pass legislation to enable a plebiscite, which will — as ABC election guru Anthony Green writes — invoke the provisions of the electoral and referendum acts.

The legislation would specify the way in which the ballot would be handled, and most importantly the question to be put to the public.

There have only been three plebiscites in Australian history — two relating to conscription during World War I, and one to choose a National Song in 1977.

Will it go ahead?

The Coalition can pass the legislation through the House of Representatives, using its slender majority of one vote. But it is the Senate that poses the problem.

The Liberal and National parties need to convince nine senators to support the legislation.

Labor has threatened to block the bill, while the Greens have promised to oppose it.

The Nick Xenophon Team of three senators also will not support the plan. Crossbench senator Derryn Hinch has also ruled out his support.

In a numbers game, that means the plan is dead if the Opposition follows through on its threat.

One Nation Senator Pauline Hanson, whose party holds four crossbench positions, wants a plebiscite but has specified it should be pushed back to the next election due to cost.

Remaining crossbenchers Bob Day, Jacqui Lambie and David Leyonhjelm all support the plebiscite.

Coalition frontbencher Simon Birmingham has warned there will not be a free vote in this term of Parliament if the plebiscite is blocked.

What will Labor do?

Without the Nick Xenophon Team's support for the legislation, the Government cannot rely on the crossbench, leaving the casting vote up to Labor in the Senate.

Labor is holding off on a final decision but has been raising serious concerns about a plebiscite through the Opposition Leader Bill Shorten and other senior MPs and senators.

The party has also been privately briefing reporters that it will eventually block the legislation.

But there is a political hitch because once a plebiscite is off the table, the Labor Party is unlikely to be able to force a free vote of both houses in its place.

There are fears that could punt the issue off to the next term of parliament, ensuring there is no prospect of gay people being able to marry before the next election.

What does it mean for voters?

If passed, Australians will be required to vote on the issue — the Electoral Act and the Referendum Act specify compulsory voting.

It does not have to operate in the same way as a referendum on the constitution, which requires a majority of voters in a majority of states.

The matter would then return to Parliament, with MPs to be guided by the outcome of the national poll.

There is a problem though — the result is not necessarily binding on MPs.

A number of Coalition members say they will still vote with their conscience on the matter, regardless of the outcome in their electorate or state.

The timing and cost

The estimated cost of the plebiscite is $160 million.

There is also a debate raging about whether any public funding would need to be released for the "yes" and "no" vote campaigns.

With regards to the timing, a spokesperson for the Prime Minister said the AEC had given advice that "strongly recommended against the conduct of a plebiscite this calendar year".