Logan Park High School science and biology teacher Yvonne Caulfield was walking near Kaikorai Stream on Thursday when a friend's dog became "interested in what appeared to be a helmet in the long grass".
The dinner plate-sized object, since identified as a red-eared slider turtle, was in good condition and may have been in long grass near the Kaikorai Valley College to lay eggs.
"It was a hot day. Five minutes after we brought it home and placed it in water it laid two eggs.
"But it later bit the eggs in half, maybe because of the stress," she said.
The turtle, now named "Shelley", is residing in a tub of water placed in Ms Caulfield's bath and is enjoying a diet of lettuce and rocket.
Ms Caulfield, who had already adopted a tropical fish collection, which is kept in her classroom, is looking for a suitable home for the turtle.
"My partner wants to keep it, the cat doesn't know what to make of it, and I'm having a baby in April, so I don't feel like I need to take on a turtle as well."
Department of Conservation acting coastal Otago area manager Matapura Ellison said the turtle was an unusual find, and the department would like to hear from anyone who had seen turtles in the area.
"[Sightings] could indicate a population is establishing, which would be a concern for the department," he said.
The exotic turtles were not classified as threats to native species, but would compete for food.
Otago Museum living environments co-ordinator Scott Kerr said turtles were sometimes released when they outgrew tanks.
"They spread through the pet trade and are bought when they are the size of $2 piece, but end up the size of dinner plates and outgrow aquariums.
"They can live for 30 to 40 years, so it's a big commitment.
"Some people take them outside on a sunny day to get natural sunlight, which is good for them, and they have disappeared."
The turtles were "extremely adaptable" omnivores, capable of surviving in various habitats, were good breeders and could hibernate in cold climates.
Shelley may have eaten her eggs for protein, he said.
Museum facilities were at capacity, so another home for Shelley would have to be found.
Red-eared sliders Trachemys scripta elegans are semi-aquatic turtles native to the southern United States. Popular as pets in North America and Europe, they are banned in Australia.
