They'll be the first gay couple to marry, but their time together is running out.

The momentous decision to (finally) legalise same sex marriage in Australia was likely met with relief and laughter by Cas Willow and Heather Richards, who’ve been together for 17 years.

But it was a bitter-sweet moment, too.

The pair will be married on Monday – making history as the first same sex couple to do so, The Age reports – but Cas might not live to see their first wedding anniversary.

At 53 years old, Cas has breast cancer that’s metastasised in her brain.

The terminal diagnosis was handed down only days before the world learned Australia had voted ‘yes’. Cas and Heather were engaged as soon as Parliament made it official on December 7.

“We asked one another and lit a little candle. It was beautiful,” Cas said.

Although a one-month notice period is typically required for couples to marry in Australia, lenience is given for couples in extraordinary circumstances.

Cas and Heather’s are certainly extraordinary circumstances.

The mother-of-three (Cas was married at 16 to a man, even though she knew she was gay) tried everything to fight the illness.

She had a double masectomy, her ovaries removed, chemotherapy. For a time, she thought she had beaten it. Only for the cancer to return, in the most heartbreaking of circumstances.

“I had to really get my head around it,” Cas told The Age.

“I knew I had to keep my eye on it for the next couple of years, but I thought I had time.”

Australia voted yes, so what next? Post continues below.

The couple’s married life will begin on Monday. Heather said it will end years of “playing house” and means she can carry on her wife’s legacy after she’s gone.

There is no time to think about what should have come before: That Cas and Heather should have been free to marry a long time ago.

Because on Monday, the pair will finally say ‘I do’ and, as Cas is losing her voice, their vows will be exchanged beautifully, and tragically, just in time.

Congratulations Cas and Heather for Monday. We wish you every happiness.

Read their full story in The Age.