The answer is "exactly the same thing as would happen if the refinery remained open", because it runs on imported oil, and the supply of that is just as vulnerable as a supply of imported petrol.
Which highlights the core problem: a huge part of our primary energy supply is imported. It also points at the solution: insofar as there is an energy security impact, it can be mitigated by reducing our reliance on oil. And in the current context, that means moving to EVs and biofuels as quickly as possible. The more of those we have, the less we have to give a damn about what happens in the Middle East, or what bullshit power games are happening between the US and China. Eliminating oil gives us actual security, and frees us from all those dangerous entanglements built around protecting its supply.
Viewing oil refineries and fossil fuel infrastructure as "strategic energy assets" is twentieth century thinking, and part of what got us into this mess. Now, we need to recognise that they're just deadweight, a 1.2 million ton a year source of carbon we can no longer afford. In the future, our "strategic energy assets" are going to be local and distributed: solar panels and wind-farms everywhere. Security of supply is built into that model, because its not dependent on a single point of failure, and you can't stop the sun from shining or the wind from blowing. Marsden Point's big shareholders can see that future coming, which is why they're getting out. What's weird is that so many other people, including people ostensibly paid to think about the future, can't.
Update: And its done! Polluting carbon capitalism looked at the future, saw it had no place in it, and gave up. Which is a tremendously positive sign.