A citizen scientist has discovered one of the world's rarest species of mangroves growing on the outskirts of the Cairns CBD.

Listed as rare and endangered on the International Union for Conservation of Nautre's (IUCN) Red List, haines orange mangroves have never before been seen in Australia.

Until this discovery less than 200 plants had been recorded worldwide.

Cairns resident Hidetoshi Kudo identified the haines orange mangrove while searching for another species of mangrove that had eluded him for three years.

"When I found it I was totally surprised because it's not on my list and not in my plants book," Mr Kudo said.

Mangrove scientist Dr Norm Duke, from James Cook University, said the species had only previously been recorded in Singapore, on the Malay peninsula and in Papua New Guinea.

"What [Mr Kudo] has found is a patch of about 25 or more trees, and one of them is actually quite large," he said.

"It's really quite remarkable and, I have to say, a triumph for citizen scientists."

Mr Kudo said the largest of the trees he discovered measured approximately 60 centimetres in diameter.

"The tree's been around for more than 100 years and Norm Duke told me it could be up to 200 years old," he said.

Mr Kudo said the distinctive colour of the haines orange mangrove flower was what led him to realise the significance of his discovery. ( Supplied: JCU )

No funding restrictions for citizen scientists

Because Mr Kudo was doing something he enjoys, he remains humble about his part in the discovery of the species.

"I was just playing a game to look for as many mangrove species as possible in the Cairns region," he said.

"I'm just doing it for myself, just for fun, and I can do it when I take my dog for a walk or anytime I like."

Dr Duke said it was amazing that so many of the rare trees had remained unnoticed in such a populated place, but not surprising that Trinity Inlet was home to previously unrecorded species.

"It highlights our sad lack of knowledge about even something as obvious as such sizeable tree species," he said.

"But I guess I've recognised that I can't be everywhere — and that applies to other people like myself who are trained as scientists.

"We manage to get some funds to do some things but we can't always get funds to do [everything].

"Before this discovery, if I was to propose to do a botanical exploration of the mangroves of Trinity Inlet that would have just been laughed at because they'd say 'you already know everything there'."