Skate egg case.
Looks very like Arhynchobatidae Arhynchobatis in TeAra, but deep water, rare?
Next best Arhynchobatidae Bathyraja but length/width ratio slightly off?
Else Rajidae Dipturus, common coastal, size is good, but photos not quite convincing.

Near new jetty's outer end(with a juvenile goatfish below Subject).Moderately common this dive,but less approachable than the species usually is.
NOTE:-Incoming tide,overcast,with unexpectedly poor horizontal visibility of 4-5 m [unexpected given,season,absence of wind,swells below half metre for several days,medium tide flux and already 3-4 days since last onshore weather event,a moderate NW gale accompanied by minimal rain,no sig catchment flows,and no sig dust storms around GSV. Possible factors making the viz so poor include(in no particular order) #1;a phyto-plankton bloom following the unusually warm summer-but viz was uniformly poor throughout water column, making this unlikely. #2:Overlapping with #1,the usual seasonal appearance inshore and intertidally of masses of shed (dead/senescent) Posidonia sea-grass blades would potentially increase eutrophication when combined with warmer than average shelf waters #3: Lingering effects of the horror bushfire season,which deposited much ash and other terrestrially sourced organic debris into both our Gulfs.Mean particle size may have been so small that swells and tides associated with even mild wind events could leave particles suspended for long periods((and only days ago,the tidal flux was large,e.g low tides of ~0.3 m and highs of ~2.5 m on any given day were the norm))

4: Widely distributed raised silt from the euphemistically named 'dead zone'* in ~middle of GSV [*a term intended to mean that the site choice for dumping the dredging's finer particulate spoils, 20 km off Port Adelaide in ~25-40 m depth ,where several million tonnes of dredge tailings were dumped last year during widening and deepening of the port's main shipping channel, is an area minimally impacted by tides and weather-seasonal or unseasonal. Which is a matter for more robust debate than occurred,I brazenly suggest(!) [Personal opinion only].

Hopefully not a re-submission. A very common but often overlooked inshore fish in many parts of SA particularly the gulfs.This one's @ 'hotspot' off jetty precinct's kiosk (NB: there are several tiny=juvenile grass cling-fish aligned along stems quite near subject, however I don't expect most viewers to confidently accept their presence.The most obvious and sharply focused cling-fish is directly below the weed whiting's abdomen, and I am 100% sure it is not artifact. I've often seen these tiny, very cryptic fish move short distances along sea-grass stems, and they often switch sides on a stem then peer back over an edge,making them very hard to photograph!)

An aggregation of adults of this species near canopy algae next to an old tree stump roughly in the middle of what I call 'syngnathid hotspot',a mainly sand and scanty seagrass area off kiosk and NSLSC.

5 images all taken within a minute or so on a dive @ 'hotspot' Normanville. (Further images to follow which hopefully will inform the topic of host/client cleaning behaviours of some species of temperate inshore fishes)
The 1st 3 images show quite a few Slender Weed Whitings loitering at edge of canopy brown algae (near the old tree stump @ 'hotspot' near Normanville jetty).
I suspect this is a part-time host species.
Some images show why I suspect this, but none prove it.
The last 2 pics include a lone S. attenuatus in foreground, and a young Magpie Perch seems to adopt client pose.

The 2nd of 3[] related submissions. All are sequential images, taken within a few minutes, on a single dive at 'Hotspot' near jetty.The 1st 2 of the 3 images in this submission again show the same juvenile Magpie Perch that is present in the 1st submission, but the S. attenuatus is now posing higher up in the algal canopy and the morwong is facing it and remains in overt client pose.(Not proof of host being S. attenuatus, as both fish species could be clients of an unseen host, but one wonders...?!)
In the 2nd image a third fish - a large male Gulf Pipefish (Stigmatopora narinosa) - shows itself (to L of the Magpie Perch) as if it also seeks parasite removal and is queuing or wants to jump the queue.
NB:of passing interest, an adult female S. narinosa can also be seen (fainter but definite upon zooming, and females have more slender snouts than males) in background on R in 3rd image.
[*IMPORTANT NOTE:The next/final submission will appear under 'Gulf Pipefish' not Slender Weed Whiting]