A few thoughts here.
Australia is a massive nation in size, in fact it's coastline is longer than the coastline of the USA even thought Australia is physically somewhat smaller. That coupled with a low population and for Twilight: 2000 timelines it would mean less than 20 million spread across the nation (probably closer to 18 million or less at that time).
Most of these people are found along the eastern coast as are many important industrial centres. In harsh reality, there's probably less than 25 important places to attack with nuclear weapons, some of these have already been mentioned.
So Australia could be significantly damaged with very little effort on behalf of the USSR including smuggling in teams and gear. As noted by Legbreaker, the vast expanse of the northern reaches are sparsely populated and some towns are as far apart as 200-800km from each other.
However, that decentralization has some benefits in that even with the complete destruction of one of those towns, it would not be sufficient to halt that State let alone the nation. The flip side of course is that that town is isolated from help should anything occur and in some cases, harbour towns being closed down would significantly affect trade etc. etc. for some time. For example, simply sinking a laden container or bulk carrier ship in the mouth of some ports up north would lock up that harbour for months simply because the heavy gear needed to clear it could take weeks to arrive (because of the distances involved).
As for sabotage teams wanting to get at least 100km clear of a target, in Australia it can be somewhat different. You would want to either stay close or get the hell out of Dodge by a good couple of hundred km's.
This is because in the country regions, the people are so isolated they tend to pay much more attention to things than city people expect. Any small town in the outback probably has a handful of people who are very aware of all the "newcomers" to town and all the 'comings & goings'.
If 'that nice young couple from Europe who'd been in town for the last few years' suddenly were absent after a local disaster/act of sabotage, somebody would likely notice and report it for fear that they had been injured etc. Obviously that could seriously compromise a special forces team trying to live in the area.
If leaving the area, you would want to get more than 100km away because in many of those towns, people regularly roam up to 100km+ away from town even in a day for various reasons (work, recreation or simply 'getting out of town for a while').
Again, these people pay attention to others in the area particularly because they may need assistance if anything goes wrong. Like any small community, people tend to recognize others from the town even if they don't actually know them so strangers will be spotted very quickly.
Any team wishing to hide out in the outback needs to be almost 100% self-sufficient because of these factors. And one other thing they absolutely must have - water, because if they want to camp out near any water source, they are going to be sharing it with all the other people in the area (again increasing their chance of detection).
You could travel for a thousand kilometres and not see another person in some places but then you would also be too far away from any normal source of food and water let alone tech/mechanical/medical assistance.
As for railways and roads, many areas of the outback are flat and road & rail can easily avoid having to go through tunnels or ravines etc. and bridges are few & far between so the chances of using these as chokepoints are much lessened. It's far easier to destroy the road/rail. Planting landmines on the road would be far more effective.
Or if it comes to that, it's easier (and probably safer) to contaminate a town's water storage.
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