I think the biggest vanes growing on this pinna shell are probably the suggested taxon. That said, I didn't see any larger or more obvious or typical colonies on any of the jetty piles or jetty junk, such as we find at Rapid Bay jetty. So if the above image is confirmed as being Steginoporella chartacea then I suspect it would be near the upper limit of Spencer Gulf in distribution terms, at least wrt inshore occurrences. (I speculate thus because I gather this species favours moderate to high energy relatively exposed sites).