A Canberra Christian couple who have vowed to divorce if same-sex marriage is legalised may not be able to follow through with their plan due to the legal prerequisites for divorce.

Director of the Lachlan Macquarie Institute and former ACT director of the Australian Christian Lobby, Nick Jensen, said he was prepared to divorce his wife of 10 years in protest if same-sex marriage is legalised by Federal Parliament.

Mr Jensen said he planned to continue living with his wife Sarah and be married "under God", but divorced in a legal sense.

"It's not extreme in our eyes, it's simply a natural consequence of our conscience," he said.

"When we signed the contract 10 years ago we made a contract with the state about what marriage is, which was husband and wife, fundamental order of creation, part of God's intimate story for human history, man and woman for the sake of children.

"So if the state then goes and changes the terms of that contract, then that's something we can no longer partake in, it makes the contract null and void essentially."

But the Family Law Act states that an application for divorce will only be considered if the parties had separated and lived separately for a continuous period of at least 12 months.

Family lawyer Denis Farrar said the court also needs to be satisfied that there is no reasonable likelihood of cohabitation being resumed.

"It surprises me that any sensible person would make such a threat, because you can't get a divorce unless you've been separated for 12 months," he said.

"It doesn't sound like Mr Jensen could truthfully swear that those facts are correct. And in that case he may apply for a divorce, but he won't get it unless he lies."

'It would be ironic if we could not divorce'

Mr Jensen conceded the Family Law Act may throw a spanner in the works.

"There will be issues around the fact that we would have to prove that our marriage was irreconcilably broken, which it isn't," he said.

But Mr Jensen said he and his wife could look at a legal challenge, which would leave them effectively fighting for their right to get divorced.

"It would be quite ironic if the state wouldn't actually allow us to get divorced," he said.

"If a group of people got together and said 'actually, we want to take this to the courts', then maybe that would be something we'd be involved with.

"It's something we take very seriously, and we feel in our conscience that we can't actually partake in any redefined marriage."

But Mr Farrar said the couple would not have a leg to stand on in any court challenge.

"The Marriage Act covers marriage, the Family Law Act covers divorce. And there is one and only one ground for divorce," he said.

"It's not a question of a contract with the state, it's not even a contract between the husband and the wife."

Mr Jensen said he believed the prospect of divorce was still far off, and he was focused on preventing marriage equality from becoming law.

"It's not a protest in my eyes, we're not just threatening to do this. It's just something we're explaining is the consequence of any change in legislation," he said.