Whoever has thought up this plan would seem to have not consulted a nautical chart. Spencer Gulf is quite shallow. Infact, there is verrrrrrry little deep water. Check out this chart here: http://fishing-app.gpsnauticalcharts.com/i-boating-fishing-web-app/fishing-marine-charts-navigation.html#11/-33.0655/137.8567
(note that you may need to change depth to metres and that depth is lowest astronomical tide)
There is a very long narrow channel leading into Port Pirie. Surrounding that channel out to some distance are shallow waters that are generally around 4 metres depth. Out a bit further are a few deeper holes around 15 metres depth. Those depths are barely adequate for small coastal ships. A Panamax ship (50,000/80,000 tonnes) have a draft of about 12 metres. There is not much deep water until a little bit south of Whyalla and even then, only about 20 metres. The chart above shows four anchorage swing areas off Whyalla. Wallaroo is also quite shallow. The channel is about 10 metres depth into Wallaroo.
There is insufficient water depth around Port Pirie, Wallaroo and Port Bonython to operate even Panamax ships. Cape Class are right out!