Four Corners "Big Fish" This monday, page-58

Yes you are right about dissolved oxygen being key, Macquarie Harbour had lower DO at depths 10m and below (<5mg/L)
Having said that, even during summer when surface temperatures exceed 20 degree Celsius, the fish have been recorded in numerous studies to dive 3 to 9 metres to a temperature range between 13.5 and 16 (this is during summer)

Now Dissolved oxygen recharges during atmospheric events or heavy waves. What MH lacked last summer was exactly that. But as you already know, the past 6 months saw plentiful rainfall and Tassal said they have experienced a significant recharge in DO in MH.

Stocking density remains key to keeping your fish healthy when DO levels are low. As I mentioned in the first paragraph, the fish will dive to a level where there is a good balance between temperature and DO. So Tassal has been and will always reduce stocking density during summer in MH in order to reduce the competition between salmon for that optimal space.

Ninja Tuna of the Bering Sea