A parenting influencer from Byron Bay has announced her fourth child's unusual name on Instagram, and it's so uncommon it might actually be illegal.

Claire Alexander-Johnston, who goes by jetsetmama on Instagram, shared her fourth pregnancy with her 116,000 social media followers and was excited to announce the birth of her newest son last week.

But when it came to picking out a name for her baby boy, things took a little longer than expected.

Alexander-Johnston's three older children already have incredibly unique names, so there was no chance this bub was going to be dubbed something simple like "Michael" or "James."

With brothers Atlas and Everest, plus sister Zephyr to contend with, this baby boy would need a name to match.

But when Alexander-Johnston announced her latest addition as Citizen Sage Alexander-Johnston 10 days after his birth, fans were a little perplexed.

The name is certainly unusual, but here in Australia it's also possibly illegal.

Briths, Deaths and Marriages consider certain names unacceptable and therefore won't allow babies to officially be named things like swear words or names that include numbers or symbols.

They also typically refuse to allow children to be given names that are considered titles, such as 'Commissioner', 'Bishop' or 'Queen' – a category that 'Citizen' may fall into.

It's an issue that has already been raised in the Alexander-Johnston family according to the Instagram post announcing Citizen Sage's name.

"Sorry this naming business has taken so long! We had a bit of a curveball when we announced your name to our family, as someone pointed out, it's possibly illegal to name you that in Australia," Alexander-Johnston wrote in the caption addressed to her youngest son.

"We tried to change it. 'Disco' even came back into play! But nothing else felt right for you as a Libra, with a (very sensible and grounded) Capricorn moon."

"So Citizen you are, and always will be- a Citizen of the world."

She also joked that if little Citizen had any issues with his name in his teen years, he could always go by the nickname 'Zen' while "hanging in the garage punching bongs with 'Benji and the crew'."

Though the name may be a little unusual, the mum-of-four is clearly besotted with it and her new little boy, her fans praising her for sticking to the name she knew would suit her son.

"I'm already so proud, and so in love with you Citizen Sage. You complete me. Welcome to the family, we're so glad you're here," Alexander-Johnston concluded her caption.