In the same-sex marriage debate, the No side argues same-sex couples already have all the same legal rights as married couples.

But for at least two same-sex couples, that isn't the case.

Toni and Mel

Toni and Mel ( Supplied )

For the 21 years they were together, Toni Irenyi-Brunne and her partner Mel dreamed of getting married.

"Mel got down on her knees and asked, 'Will you marry me?'," Toni told 7.30.

"We knew we couldn't do it at the time but we had a commitment ceremony."

They had their first commitment ceremony in 1998, and a decade later they renewed their vows.

The couple were still hoping that one day they'd legally wed, and had gone as far as to pick out their wedding outfits.

But a tragedy in mid-2016 shattered that dream.

"I woke up to hear Mel screaming and she was having a seizure in her sleep," Toni said.

"She was taken to the hospital and diagnosed with a brain tumour."

Mel's health deteriorated quickly, and she died just three months after the diagnosis.

Toni's trauma of being suddenly widowed was compounded when she received Mel's death certificate.

There was no mention of her having a relationship with Mel, although Mel's ex-husband, who she separated from more than 20 years earlier, was listed on the certificate.

On the death certificate Toni was simply listed as the informant — the person who provided information about the person who died.

"To me it makes me feel like our relationship means nothing, that I was invisible," she said.

"Over 12 months later it still hurts. It hurts a lot. I don't want that to happen to anyone else."

Joe and Greg

Joe and Greg in 2003. ( Supplied )

Joe and Greg have been together for 14 years. In 2011, on the same day that he retired, Greg was diagnosed with Parkinson's Disease.

As soon as they received the diagnosis, the couple decided to put their affairs in order.

But when researching Greg's superannuation scheme they realised their relationship was treated differently to married couples.

"It's constructed around statutory declarations, and it's really detailed criteria that go on for several pages, of the areas that you must provide statutory declarations about," Greg said.

His partner Joe says there's a "hugely onerous" burden of proof to prove their relationship.

"We didn't really realise the depth of documentation required because we don't have a civil marriage certificate," he said.

"It was just appalling how many things were different.

"They were asking when we started having a sexual relationship, and what our division of finance was and household chores."

The couple have been building up a brief of evidence to prove their relationship and on the advice of their financial planner they got married in the United States.

"She [the financial planner] made a comment that it was a shame that people had to go overseas to get married," Joe said.

"Greg asked her if she thought that would make a difference to our circumstance here and she said it was very compelling evidence.

"So it was kind of sad that we planned our wedding so that we would have compelling evidence."

Joe and Greg are frustrated when they hear opponents of same-sex marriage argue de facto couples have equal rights.

"This is just flat out not true, we do not," Joe said.

"We have a lot of extra hoops that we have to jump through."

Coalition for Marriage: It's already a level playing field

Lawyer for the Coalition for Marriage, Christopher Brohier ( ABC News )

Campaigners opposing same-sex marriage believe there is already sufficient legal protection for de facto couples.

Barrister Christopher Brohier from the Coalition for Marriage says a number of legal changes mean same-sex de facto couples are on par with married couples.

"Starting in 2005 there were changes to Commonwealth legislation, and then the big ticket items happened in 2008," he told 7.30.

"The big changes to superannuation for example, and that effectively created a level playing field.

"You can look at then end of life issues, and there are stock-standard provisions which everyone can and probably should access — will, advanced care directive, enduring power of attorney, enduring power of guardianship and similar issues.

"That takes care of the issues of next of kin."

But Hannah Robert from La Trobe University Law School disagrees.

"I see this bandied about, 'you're already equal'. And you know technically in many areas there is that technical equality," she said.

"But when it comes to accessing your rights, that's not necessarily the case.

"You've got different definitions of de facto in every state and territory, you've got different definitions of de facto, of domestic partnership or domestic relationship for different pieces of legislation.

"So the goal posts shift according to what you're trying to do."