Visitors and residents on the island are being urged to stay away from the popular tourist area as rangers carry out a search to confirm reports.

Straddie Adventures owner Mark Jones said he saw the crocodile near Myora Springs, just a few kilometres north of Dunwich, while on a kayaking tour with nine customers on Wednesday morning.

He said rangers had closed the area and were advising people near Amity to stay out of the water.

"I actually thought it was a log to start with, but that changed within a few seconds because it actually went under the water," he said.

"I said to my customers 'just watch in this direction, you will see some ripples'.

"One of the followers had seen it as well, and then it popped up again. I said 'that's a croc' and they agreed."

Mr Jones said he had seen sharks, dugongs and turtles on the island before, but never a crocodile.

"It just stayed outside the mangroves and was heading north in the Amity direction," he said.

"My customers ask me 'do you get crocodiles here?'. I say no, we don't, because if we did I would be doing my tours in a boat."

The Environment and Science Department said crocodile warning signs were being installed at boat ramps on the island as rangers carried out their investigation.

"Rangers will conduct vessel-based searches and use a drone to search the area," the department said in a statement.

"Visitors and residents on Minjerribah are urged to stay away from Myora Springs for safety reasons, and to allow rangers to complete their investigation.

"The investigation will identify if a crocodile is present on Minjerribah."

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Mr Jones was not put off by the sighting, jumping into the water to spear a flathead near where the crocodile was seen during the same tour.

North Stradbroke Island is classified as Zone F under the Queensland Crocodile Management Plan, meaning any crocodiles found on the island would be removed from the wild, regardless of their size.

"Members of the community can report crocodile sightings by using the Qwildlife app," the department said.

Mr Jones said he rang police to get the word out and then had his partner ring authorities to report the sighting.

He believes the crocodile was about two and a half to three metres long.

"They have closed Myora Springs and gone down to Amity to advise people not to jump in the water," Mr Jones said.

"I haven't heard of anyone sighting it again, but we will see what they [rangers] are doing."