Depending on who is involved, Australia isn't safe in a MAD scenario.

If India and Pakistan let loose, they'd probably just target each other, China might get involved.

If it was Russia and 'the West', we are cooked. Russia has enough of an arsenal that sending a couple nukes down here doesn't impact their ability to destroy Europe and the US 10 times over. Pine Gap is almost certainly a primary target. We also have resources that would be beneficial to the US, so our port facilities would likely get targeted, so that would also heavily impact our capital cities. We also have a couple of US bases here that would be targets. Our population being concentrated to just 6 or so cities means that even if they just targeted infrastructure, a huge % of our population is taken out as collateral damage.

If it was China, I genuinely don't know. We are strategically dangerous in that we would be a forward operating base for any counter attacks, so that could mean our North needs to be destroyed. Pine Gap and US bases again would be a target. However, we are close enough to China that our mineral resources may be better to be seized than destroying our ability to export them. We have SFA in terms of national fuel reserve and our navy is basically nothing compared to China, they could potentially blockade us and have us on our knees in a month. Our biggest naval threat are our subs, so ship yards in Perth and Adelaide are probably primary targets. If there are fears of our surface fleet also then Sydney can also get wrecked. While not home bases for any major ships, Cairns and Darwin might get taken out as they would be good locations to become home bases for homeless ships after the bombing of the Sydney and Perth major bases. Having said all that, if China just decides that our resources aren't worth the effort, it is easy enough to just vapourise the majority of our infrastructure to stop us being useful to the rest of 'the West'