A Sydney mother has shared a warning after her son had a scary reaction to a henna tattoo while on a family holiday in Indonesia.

Kerrie-Anne Graf told 9news.com.au she and her 13-year-old son were at Kuta Beach, on the south-west coast of Bali last month, when they both made a spur-of-the-moment decision to get a henna tattoo.

The henna tattoos worked out great and were a big hit with her son, Graf said.

However, a few days later, the pattern had started to fade. 

So, when the family was at Seminyak Beach and some locals came past touting henna tattoos for $10, her son asked if he could have the tattoo on his arm redone, Graf said.

“These guys just walk up and down the beach and they’ve got their book with dragons.

“You’re there with your kids and they’re saying, ‘Mum, I want a tattoo’.”

Unlike the first henna tattoo she and her son got, which was a traditional brown colour, the second tattoo on her son’s leg was black, Graf said.

“He got it done the day before we left Bali and it seemed fine,” she said.

“Then he started itching at it and, maybe two days later, it just blew up.”

Graf said her son’s arm became extremely swollen and developed a rash.

“At first, I just thought maybe it was nothing serious. But then the rash started to spread up his arm,” she said.

When her son was sent home from school because his arm was weeping, Graf took him to the GP, who prescribed antibiotics.

“They did nothing, so we went back and he got a double dose of really strong antibiotics,” she said.

In addition, Graf said her son needed to take steroid medication, antihistamines and apply cream to his arm.

“I’ve spent over $140 on medication,” she said.

Graf said she wanted to warn other families travelling to Bali about the risk of henna tattoos.

After making a post on a popular Bali travel Facebook page, Graf was inundated with comments from other Australians who had suffered similar reactions to henna tattoos in Bali.

At the time, 9News found the “black henna” used by some operators in Kuta was made from hair dye containing paraphenylenediamine (PPD), a chemical that as many as one in five people are allergic to when it’s applied to their skin.

Graf said she believed it was this black hair dye that was applied to her son’s skin.

“It shocked me because I had never heard of that happening.”

The mum said she was still very worried her son could be left with permanent scars.

“I can’t put anything on it yet, like Bio Oil, because I think it will just irritate it more.

“So I’m just going to wait, and we’re probably going have to see a dermatologist.”

Graf she said hoped sharing her son’s experience would make other tourists think twice.

“I wanted to say something because I don’t want this to happen to any other kid,” she said.