I've taken a guess at these being a marine invertebrate because there is a part of me that thinks they are foam packaging bubbles. Many found scattered over Bunga Beach with some still floating within rock pools and at the waters edge. They were mostly white and in varying degrees of flatness. The largest was approx. 15 cms but the majority were between 5 and 10 cms. They were mostly around 2 cms wide and there was a hole in one end. They were rubbery in texture and strong. I found a select few that were still plump. Those were hollow but easily flattened.