This sea slug would have been about 5 " in length. It had mottling of cream and brown. The ear-like sensory clubs or rhinophores and oral tentacle were withdrawn and the usually extended side flaps called parapodia were flipped back. The mantle and atrophied shell could be seen (pic 5) as a reddish radiating structure.
I have the expert Mattt Nimbs to thank for the ID. He says "Definitely Aplysia. And yes probably A sydneyensis, there is a wheel like pattern of radiating stripes on the mantle that sits over the vestigial shell: a distinguishing characteristic of sydneyensis"