As part of the current restoration work at the wetland, about 70 oblong turtles were taken to a nearby fenced enclosure, where City of Bayswater rangers believed that they would be safe.
Mayor Sylvan Albert expressed his shock that so many of the turtles had disappeared.
�It would appear someone has gone to great lengths to get to, and illegally capture, the wetland turtles,� he said.
�Notwithstanding this, we are also looking into the possibility of predation, with extra on-site measures to prevent theft and predation now implemented.
�The turtle population will take a long time to recover from this.�
Cr Albert has asked residents to be vigilant to help prevent any potential further thefts of wildlife.
�If you have seen or do see anything suspicious around the bird sanctuary, contact our Ranger and Security Service on 9272 0972,� he said.
The incident was reported to the Department of Parks and Wildlife.
A department spokeswoman said it was conducting an investigation into the alleged theft of the turtles.