Guam is essentially a military base already. Its US territory, has i dunno 10,000+ US military personel. Carriers, USMC amphibs and the USAF often base out of Guam. It would be akin to trying to attack a huge military staging point on US soil.
Australia's communication stations are very remote, and include things like echelon stations etc. Most of the important bits are fortified, and behind multiple fences km away from the actual sites. Usually manned by US and sometimes US and Australian personel. These sites probably have as their biggest threats all out nuclear attack by ICBM's, because thats the level you would have to go to take them out.
Terrorists would find a lot softer target than that. Usually civilian or troops moving about in unarmoured humves.