In August 2015, the Abbott government committed the Coalition to a plebiscite on whether Australia should enact marriage equality.

Since the election, Prime Minister Malcolm Turnbull and Attorney-General George Brandis have promised the vote will take place soon. But does Australia need to have a plebiscite? And if we do have one, how should it work?

Constitutionally unnecessary

This plebiscite is constitutionally unnecessary. The High Court ruled in December 2013 that the federal parliament has the constitutional ability to pass legislation providing for marriage equality. There is no need for constitutional change in Australia, unlike in some other countries such as Ireland.

The High Court also made clear that parliaments have changed the legal understanding of marriage, and of what marriage entails, many times in Australia’s history.

It is parliament’s responsibility and within parliament’s power to legislate on marriage. This is not surprising: Australians expect parliament and government to make significant and controversial decisions about the budget, the tax system, the country’s defence and much more. There is no reason for marriage to be different.

The plebiscite will therefore be an unnecessary and costly vote, with potentially damaging consequences for many Australians. It has, nevertheless, been promised.

Designing plebiscites

Under Australia’s constitutional and legal arrangements, there is no set formula for how a plebiscite should be organised. Australians make a distinction between referendums, which are national votes required to amend the Constitution and which must be conducted in a particular way; and plebiscites, which are national votes for other purposes.

Australia has had just three national plebiscites: two on conscription of troops during the first world war, and a 1977 vote on the national anthem.

It seems likely parliament will need to pass the framework legislation for any marriage plebiscite. So how should this plebiscite be designed?

When designing the plebiscite, the parliament should bear in mind Britain’s recent vote on leaving the European Union.

One of the truly remarkable features of the Brexit vote was that it took place without any clear proposal on what would actually happen in the event of a vote to leave the EU, and when it would take effect. As a result, more than a month later the British people and government are still figuring out what “Leave” means.

If a national vote is worth having on marriage equality, the Brexit trap must be avoided. Australian voters need to know what they are getting if they vote “Yes”; they should know their vote will have an effect and what that effect will be. If a national vote is worth having, it should be more than an “advisory” in-principle poll.

Avoiding the Brexit trap

So, here is a proposal for the plebiscite’s design. While it may not be perfect, it will help Australia avoid the Brexit trap.

The federal parliament should pass a single new law. That law should do three things:

First, it should set out precisely and in detail the proposed amendments to the Marriage Act. There are already plenty of examples of what this could look like. Existing proposals would legislate for marriage equality while also ensuring that, as is currently the case, religious ministers need not marry couples they don’t wish to. There could also be details of related amendments that might need to be made to any other legislation.

Second, it should set up the machinery for how the plebiscite vote itself will be run and administered (the compulsory or voluntary nature of voting, for example, or public funding for the campaigns). It should also ensure there is a formal process for someone – like the governor-general – to officially declare the result soon after all the votes are counted. There is no need for a complicated formula to determine which side has won; whichever of “Yes” or “No” gets the most votes, that side wins.

Third, and most importantly, the law should state that the proposed amendments to the Marriage Act do not take effect right away. Instead, the law should make it clear that, if there is a “Yes” vote, the proposed amendments would take effect within 30 days of that result being officially declared. If there is a “No” vote, then the proposed amendments do not take effect.

These three elements ensure the Brexit trap is avoided. Voters would know the proposal that is on the table, right down to the punctuation. This means the question on the ballot paper could be something simple:

Do you agree to the proposed changes to the Marriage Act that are set out in the new law?

Every voter’s choice would be relatively straightforward: “Yes” or “No” to a particular proposal.

What’s more, voters would know that if they vote “Yes”, federal parliament does not need to take any further action. In the event of a “Yes” vote, the proposed amendments will automatically come into effect soon after the governor-general declares the vote. And voters who vote “No” will understand what, precisely, they are saying “No” to.

Making the vote count

Under this plan, voters will know their vote matters, and that their vote will have an effect. But there are risks involved.

As the 1999 republic referendum demonstrated, there can be political risks in having a concrete proposal on the table. And the very nature of any plebiscite means that, at some point in the future, federal parliament could choose to revisit the result, whatever it is, and even choose to amend the Marriage Act again.

But the plebiscite would have created facts on the ground: a “Yes” vote would make marriage equality a reality soon after the vote.

Alternatively, Australia could run the risk of the Brexit trap: a vote could be designed without a clear proposal and without a clear plan for what happens in the event of a “Yes” vote. But Australian voters deserve better than that.

The plebiscite is unnecessary and it may well prove divisive. There are some doubts over whether particular features of a plebiscite may be constitutional.

It isn’t too late for the government to reconsider, nor for the opposition parties to block the plebiscite in parliament. But if it is going to happen, the plebiscite should be a chance for Australians to cast a meaningful vote on a clear proposal.