Forensic testing is underway to determine exactly why an orange purchased at a Brisbane fruit and veg shop turned purple hours after it was cut open.

Keperra resident Neti Moffitt cut up the orange on Tuesday as a snack for her two-year-old son.

It looked fine, smelt fine, and the three slices he sucked on were later binned. The rest of the orange was left out on the kitchen counter overnight.

On Wednesday morning, Ms Moffitt woke up to find the remaining fruit was part orange, part purple.

“It looks like someone’s dipped it on an ink pad, which I guarantee you we haven’t,” she said.

“I went rifling through the rubbish bin for the three bits eaten by my son, and sure enough they were more-so purple than the ones left out on the bench.

“My first thought was I hope has had no ill effect on my child. But he’s fine, absolutely not a drama.”

Ms Moffitt hit the internet where she found only one other occurrence of an orange turning purple — also in Queensland in 2015.

The orange was cut on Tuesday afternoon, and by Wednesday had gone purple. Photo: ABC/Neti Moffit

At the time Nine News reported a number of oranges, purchased at Chinchilla in the Western Downs, turned purple after they were cut open.

It said forensic testing ruled out artificial colouring, and no iodine was found.

Ms Moffitt contacted Queensland Health to investigate her case.

“The gentleman who came to collect it was very, very excited,” she said.

“He was aware of the case three years ago, and he’d spoken to the chemist who tested the orange three years ago and said ‘look, I think we’ve got another one’.”

They have taken the orange and the knife used to cut the orange, as well as a steel knife sharpener that was recently used.

“There’s only one other case that’s been reported. I posted it on Facebook and everyone has their own speculation to what it could be … but no-one’s in a field where they have any scientific proof or evidence.

“If it’s some kind of weird acid-loving mould or iodine reaction to the knife, wouldn’t that information be well known?”

Queensland Health have promised to keep Ms Moffitt in the loop on the test results.

Queensland Health have collected the affected orange, as well as some other items from the house. Photo: ABC/Neti Moffit

“I’m so keen to find out what has happened,” she said.

The purple orange was one of several purchased last week, one of which was also taken by Queensland Health as part of its testing.

“The orange also leaked some purple onto the vine of a lemon I had cut open in the fruit vine. So they took that also.”

“They were really curious with fruit trees we have in our garden. I also have a bunch of roses besides the fruit bowl, so they took that into consideration.

“But they’re baffled, they’re absolutely stumped. No-one knows what caused it.”

A Queensland Health spokeswoman confirmed an officer took the discoloured orange slices, the knife, the sharpener and other items.

“The items have been taken to Queensland Health Forensic and Scientific Services for testing,” the spokeswoman said.

Citrus Australia has been contacted for comment.

-ABC