As someone using a wheelchair and bringing my own into the park I found my front tyres would get stuck in the grates placed across the park. Perhaps using their wheelchairs would negate this problem as it looked like they may have had thicker front tyres, but that being said mine isn't all terrain.
Cleland Wildlife Park Accessibility Map. Although it may not seem so on the map, the aviaries are also accessible for some if you can navigate the 'air lock" system with 2 doors and plastic blinds. The park is very hilly and if you are unmotorised I would suggest being fit to get through this park.